Fir Audio Radon 6

Fir Audio Radon 6 is a limited flagship-level IEM by the American manufacturer. It uses a single Kinetic Bass DD driver, 4 balanced armatures and a single electrostatic driver. It's priced at $3299.

Introduction to the Fir Audio Radon 6 review

Fir Audio Radon 6 box review

Fir Audio has quickly become my favorite IEM manufacturer in the world. While reviewing their previous flagship, the M5, I said that they’re really close to getting to the IEM Summit-Fi, and they achieved it with their next line. 

When I reviewed their Xenon 6 it quickly became my favorite IEM to date, and I was sure that they’ll retain this position for a long time. Then, out of nowhere, the Krypton 5, which is actually a lower model to the XE6 came and absolutely stole my heart. My favorite IEM/Headphone ever, and it was truly worthy of winning our IEM and Product of the Year 2022.

Fir Audio really knows how to make a summit-fi IEM that sounds absolutely incredible, but not analytical and neutral at the same time. In High-End, it’s all about different flavors and original approach to the sound, at least in my opinion, and Fir Audio mastered that approach.

After the huge success of their New Frontier series, they came up with a limited model, the Radon 6. There are only 300 units in the entire world, so definitely, not everyone will be able to test it and buy it. With the price set between the Kr5 and Xe6, the Radon 6 is set to be a middle-ground of these two, providing the best aspects of both models. 

And here, I want to say that I’m very grateful that I’m able to test the Radon 6, even though it’s such a limited product. With that being said, let’s see how the Radon 6 stacks against the competition.


Box of reviewed Fir Audio Radon 6

When it comes to the packaging, things are very similar to the Xenon 6 and Krypton 5. You’re still getting the same box with great graphic design and quality. It is inside of the box that matters truly, and we’ll just get right into it.

So, the first thing that is different is the new case. It looks quite similar at first, but if you have both the new case and the old one in your hand, it’s really easy then to see differences.  The new case is slightly smaller in height, but it’s made differently, using better materials. The leather itself has a finer texture, giving a more premium look and feel. Additionally, even the bunny logo on top is now more prominent. It’s not that the old case is bad in any way, but when you’ll see the new one, you’ll immediately see that there was room for improvement.

Actually, the New Frontiers case was already one of the best cases I’ve used with IEMs, and I don’t think that Fir Audio got a single feedback to improve it. But, they just decided to do that…it’s a classic Fir Audio swag – It seems like they can always do better.

Next up, the rest of the accessories is nothing to write a book about if you’ve read our KR5 or XE6 reviews. The reviewed Fir Audio Radon 6 comes supplied with better eartips, which is always a nice thing to see, but most of you will probably end up using your favorite tips anyway. However, credit must be given when it’s due, and once again, Fir Audio just went the extra mile to assume that everyone is happy with their new IEMs. Nonetheless, adding Symbio hybrid eartips with the reviewed Fir Audio Radon 6 was actually a great idea, because these are by far one of the best eartips on the market and they actually suit the Radon 6 beautifully.

Apart from that, you’re also getting a cleaning tool, a set of Atom modules (more on that later), and a new custom-made cable. Fir Audio has always been a company that cares about the unboxing experience and the accessories you’re getting with your new IEMs. While this is by no means a luxurious type of experience, you really cannot say anything bad about it. Functional, elegant, and clean.

Design, Build and Comfort

This might get repetitive, but once again, the reviewed Fir Audio Radon 6 shares the build quality and design with the New Frontiers lineup. And to be absolutely honest, this is the best thing Fir could have gone with.

The build quality and design are both top-notch with an astonishing fit and finish, as well as a beautiful design and infinite ergonomics. Fir Audio might be the company that offers the best build quality of every single IEM on the market, and the Radon 6 is obviously no different here.

So, to begin with, the RN6 feels extremely solid in hand, it has some weight to it, but not too much for it to be problematic in terms of ergonomics. This is a great balance between pure quality and comfort, which causes absolute zero worries when it comes to the durability of your new high-end IEMs. If you’re always taking your IEMs with you, and you don’t really want to be hyper-cautious about them, the entire new lineup from Fir Audio might be the best choice for you. 

Actually, if you’ll somehow manage to ruin the connectors or the nozzle mesh, Fir Audio got you covered. Thanks to their Rigid™ technology, these parts are easily replaceable. Just sent your damaged IEMs directly to Fir Audio and they’ll have them fixed for you – How cool is that?! Obviously, I’d highly recommend caring about your $3000+ IEMs, to begin with, but it’s nice to know that if anything bad happens to them, the trash bin isn’t the only solution. Fir Audio offers a type of approach that everyone has to respect. 

On top of the exceptional build quality is the design that just looks absolutely sick. This time, Fir Audio went with matte-black shells and sapphire crystal glass faceplates with carbon fiber and gold flakes infused into them. These IEMs look absolutely sick, definitely my favorite from the entire New Frontiers lineup. It’s just such a classy design, stealthy yet interesting, minimalistic yet refined. Just look at the photo above and appreciate this design, as it might just be the best-looking IEM faceplate ever.

Additionally, as I stated before in this review, the comfort of the reviewed Fir Audio RN6 is simply excellent. I can wear them for long listening sessions with not even the slightest sign of fatigue, even though it’s not the lightest IEM I’ve tried, not even close. It’s all about that well-designed shell size and shape that truly matters in IEMs, and Fir Audio got that covered brilliantly. 

Lastly, the reviewed Fir Audio Radon 6 comes with a new, custom-made silver cable, that was built and designed exclusively for this IEM. It is a pure silver cable with copper shielding, and it’s by far one of the best-sounding stock IEM cables I’ve tested ever. Talk about attention to detail. However, the cable is a bit stiff and heavy, so keep that in mind if you are a fan of lightweight, thin cables that you quickly forget about. You’re not forgetting about this one, but the quality is definitely there. You can also choose your termination, I went with 4.4mm…for obvious reasons.


inner part of reviewed Fir Audio Radon 6

As I already mentioned, the reviewed Radon 6 uses the best technologies from Fir Audio, including the Rigid™, Kinetic Bass™, Atom Venting™, and Open Acoustics™. 

At this point, you know what Rigid stands for, but let’s discuss the other ones. 

Kinetic Bass™ is a technology that I already called a breakthrough in my Krypton 5 review here. The Kinetic Bass technology in the RN6 features a 10mm dynamic driver that is open and ported in an optimized manner directly facing the outside of the IEM shell. This design allows low-frequency sound to be transmitted directly to the ear cartilage and surrounding area.

Low frequencies are then converted to kinetic energy and reach the inner ear via bone conduction. This results in a more immersive sound with a bass you can feel, and it’s not just a theory. The bass of the entire New Frontier series is a breakthrough for the IEM market when it comes to low frequencies, and I’ll express upon that later in the sound paragraph.

Kinetic Bass makes the bass so physical, punchy, and pronounced that this is the only IEM in the world that you should consider if you’re a bass-head. There’s nothing that comes close for me personally. This feature gives that additional meat to the bone in the bass department, and the description of Fir Audio here is just spot-on, “a bass you can feel”.

Also, the reviewed Fir Audio RN6 incorporates the Atom Venting system, which helps reduce ear fatigue during extended listening sessions and at higher volumes. This system vents the pressure that builds up in a sealed ear canal, allowing for a more comfortable listening experience. On top of that, you can easily manipulate the isolation level by using different ATOM modules, which basically give you 4 IEMs in one. Those 4 different levels are:

RED : These modules give you the least amount of isolation (10dB), which gives you -4dB in the bass department when compared to the “neutral” silver module. This is the most technical, fast-sounding module, that lacks meat to the bone in my opinion. It definitely gets the least amount of listening time for me.

BLACK : These ones give you 13dB of isolation, which transfers into -2dB when compared to the neutral one, silver. It has more weight and warmth than the red module, sounding more natural and just more fun. While not being as analytical, it simply sounds more “right”. 

SILVER : The silver modules are rated as N, which means neutral. It provides 15dB of isolation and is meant to be the middle ground. This is by far my favorite module, as it sounds incredible both technically and musically. Both objective and subjective sound is fantastic with silver modules, and I’d highly recommend starting with these, and then fine-tuning if you’d like. Please note: This review is made using silver modules, for the most universal sound descriptions.

Yellow : these modules offer 17dB of isolation, which gives a +2dB boost over the silver ones. This is the most bassy type of experience you can get with the Radon 6, and having in mind how incredible the bass is while using the silver modules, the yellow module is reserved for the most extreme bass heads. It hits the hardest, the rumble is just absurd, but it doesn’t sound as refined and natural as the silver module. My second favorite for electronic music and metal, but I prefer the universality of silver modules more.

The last technology used in the Radon 6 is Open Acoustics. This system in the RN6 eliminates the use of sound tubes commonly found in traditional IEMs. Instead, the open drivers radiate sound directly into a sound reactor, which shapes the sound using natural acoustics. This approach results in a much larger soundstage and a more pure and natural listening experience.

In terms of driver configuration, the RN6 features a tribrid system consisting of tubeless balanced armature open drivers, an ultra-high electrostatic tweeter, and a dynamic driver with Kinetic Bass technology. The specific driver configuration includes a 10mm dynamic driver for bass, 2x open balanced armature drivers for mids, 1x open balanced armature driver for high-mids, 1x open-balanced armature driver with a sound reflector for highs, and 1x electrostatic tweeter for ultra-high frequencies. 

Reviewed Fir Audio Radon 6 has an impedance of 28 ohms, which indicates its compatibility with a wide range of audio sources. The sensitivity of the RN6 is not mentioned in the available information, but I’d rate it as quite average. All this makes the Radon 6 rather easy to drive, so basically every DAP will be able to handle them with no problem. However, note that these IEMs definitely do scale with better equipment, and I highly recommend investing in a high-quality DAP or DAC and AMP to use with these. Provide them with the best sound signal you can, and these will make good use of it.

How does the Fir Audio Radon 6 sound?

The Frontier series by Fir Audio is what changed everything for Fir Audio. Previously, with the M-Series, Fir Audio established itself as one of the top IEMs manufacturers out there, but it needed that final touch to fight for the top of the top place.

The M-series definitely wasn’t the first thing that came into audiophiles’ minds when thinking about TOTL IEMs. However, instead of focusing on what they’ve already achieved, Fir Audio just went back and started tweaking and inventing new ways to improve. 

You see, the new Frontiers series wasn’t just about “doing something new”, slapping a hefty price tag on it, and pretending it’s revolutionary. The amount of technology, research, and heart that went into it is just obvious once you spend even 10 minutes with either of the new models. While the Xenon 6 is incredibly fun to listen to, thanks to its warm, and incredibly thick sound performance, the Krypton 5 is the top dog in my opinion. Our “Product Of The Year 2022” is just an astonishing product in every aspect. This is the best IEM I’ve heard in my life, period.

So, Fir Audio now released the Radon 6, an IEM limited to 300 units worldwide, that is highlighting 5 years of Fir Audio. It uses all of the technologies that are present in the Frontier series and is said to sit between Krypton 5 and Xenon 6. 

Let’s start with the bass. It is tight, controlled, and extends deep into the sub-bass region. It provides a solid foundation to the overall sound, adding weight and impact to the music without overpowering the other frequencies. Whether you’re listening to electronic music with deep bass drops or enjoying the rumble of double bass in jazz tracks, the Radon 6 delivers a magnificent bass experience. I’ve said it a few times already, and I will say it again – the bass response of Fir Audio IEMs is the best on the market by quite a margin. 

The Kinetic Bass technology is absolutely incredible at providing a physical bass that you can both hear and feel, and it’s a fantastic sensation to experience it. At first, you’ll definitely see this bass as huge and overpowering, but the more you listen to it, the more you’re convinced that it’s the way it’s supposed to be and that you’ve been lacking bass energy with traditional IEMs. For many years now I’ve had a feeling that the high-end headphones and IEMs market focused on a bass response that is tight, controlled, and detailed, but it overlooked the fun factor, not giving us enough energy and power to simply headbang to.

Luckily, Fir Audio aimed to combine the TOTL qualities with a tuning that is pleasant to listen to, which was meant to give us a performance that will be both incredibly technical and enjoyable at the same time. They definitely succeeded, and that’s why I’m such a big fan of this company. There are a lot of high-end IEMs on the market, but none like this in my opinion. You’re not missing a single thing when listening to the Radon 6, you’re getting a complete experience, to say the least. Actually, after listening to the Radon 6 for some time, all of my over-ear headphones sound dull and powerless in comparison, even when plugged into the most high-end amplifier such as the Feliks Audio Envy

Actually, Fir Audio IEMs are THE IEMs that changed my perception of the current headphones market. There’s just nothing like it when you can appreciate every single detail and godlike resolution, while also being able to get loads and loads of excitement when listening to every music genre. Metal and classic rock are not exactly the genres audiophiles go for, but with the Radon 6, there’s no reason not to do it.

Moving on to the midrange, reviewed Fir Audio Radon 6 continues to impress with its clarity and detail. The midrange is well-balanced and accurately reproduced, allowing vocals and instruments to shine. The sound is just incredibly open and clean, which also extends to the fantastic sound-staging performance (more on that later).

Female vocals have beautiful timbre and shine to them, resulting in a sound that is marvelously airy and realistic. Male vocals sound impeccable as well, with great thickness and note weight. Especially vocalists with low voices, benefit from the Kinetic Bass technology the most. As I said in my review of Krypton 5, Kinetic Bass also improves the sound of the rest of the spectrum, adding physicality and rumble.
However, keep in mind that this is still a rather thick and warm-sounding midrange, especially when compared to the Krypton 5 which is more neutral and crystal-clean sounding. It’s not as warm as the Xenon 6 though, resulting in a sound that sits in between being marvelously clear, detailed, and pleasant sounding. While the Xe6 could be too dark and warm for some, the reviewed Fir Audio Radon 6 still gets a lot of its remarkable richness, but now combined with a more open, airy, and crisp-sounding mids. This is definitely a more “safe” approach to the tuning, which will definitely appeal to a larger audience. 
The treble response of the Fir Audio Radon 6 is detailed, extended, and well-controlled. The treble presentation is smooth and refined, allowing for an enjoyable listening experience, but it doesn’t hide any details at the same time. This is, again, not as safe or dark sounding as the Xenon 6, but still rather warmly tuned. There’s a lot of air in the top octave, resulting in fantastic soundstaging capabilities, which we will focus on in a minute.
What’s doing wonders with the treble of the Rn6 is definitely everything about female vocalists. Doesn’t matter if I’m listening to Tove Lo, Stevie Nicks from Fleetwood Mac, or Billie Eilish, the vocal range is always reproduced brilliantly with a timbre that sounds both natural and engaging at the same time. The sound has a sparkle to it, but it’s never becoming tiring or overly present. The level of detail is also top-notch, as you would have expected from an IEM of this price bracket. While tonality is slightly more on the “pleasant” side, the technical performance is simply fantastic. This is what you’re getting with good high-end products – a proper technical performance with a well-presented flavor profile. 
The Krypton 5 is slightly more detailed than the two, pushing the boundaries even further, but to say that the treble response of the Radon 6 lacks detail, would have been incredibly too far-fetched. The Krypton 5 is a beast after all, so it’s not as surprising. However, if you’re into a more romantic and smoother sound experience, the Radon 6 might come as superior to the KR5 for you.
The soundstage is the part of this review that I’ve mentioned a few times already. Usually, IEMs with a warmer/thicker sound characteristic don’t do too well with the soundstage, but the Radon 6 is just built differently. 
The sense of realism when it comes to the soundstage is absolutely brilliant here, with incredible imaging and infinite depth. The Krypton 5 is slightly wider-sounding, but the Radon 6 uses its warmer sound to its advantage here. Because of that, you’re getting a fantastic, black background with instruments popping out of nowhere, and combined with the Kinetic Bass that adds physicality to everything you’re hearing, this is a type of experience that is impossible to beat for other IEMs. 
It’s actually quite interesting for a warm-ish sounding IEM to have a soundstage that vast and open-sounding, with such great separation. It works wonders with every genre I’ve tried it with, starting with electronic music, and ending with acoustic live performances of Eagles. When I said that the Krypton 5 is slightly wider sounding, I obviously didn’t mean that the Rn6 is narrow-sounding, not at all. It has a great width, but just not as impressive as its younger sibling. However, when it comes to depth, I’d call it a tie, and considering how much I praised the Krypton 5, this is very impressive.
There’s just so much air in the sound of the Radon 6, but at the same time, it has a lot of weight and body to it. This is definitely not your “classic” spacious-sounding IEM, mainly because of its beautiful tuning. 
To summarize the sound of the Fir Aduio Radon 6, it is a technical-sounding IEM with an incredibly physical bass response, a slightly warm midrange, and an open, yet delicate treble. All of this is paired with a soundstage that is huge, and natural with fantastic imaging. If you’re looking for high-end IEMs with tons of details, the best bass response there is but at the same time you’d like a slightly thicker, warmer sound presentation, the Radon 6 could be just the best IEM in the world for you.




This is probably the most important comparison in this review. While the Radon 6 is a limited model, it’s priced similarly to the Krypton 5, which has won our IEM of the Year 2022 and Product of the Year 2022 awards. 

So, to answer the most important question right away – I think that Krypton 5 is a better IEM, at least for me. It is even more detailed, more open-sounding and its tuning is more natural. However, the Radon 6 is more laid-back sounding with more meat to the bone. Of course, you can alter both IEMs slightly thanks to the ATOM modules that are supplied. Because of that, you simply can’t go wrong with either.

When it comes to the technical performance, I’d give an upper hand to Kr5 thanks to its more neutral, technical tuning, but it’s not a big difference. We’re definitely speaking about minor differences, that could be irrelevant for you at this point. If you’ve tried the Krypton 5 and found it a bit too neutral for you, the Radon 6 is definitely the IEM that you should try next. It’s not a revolution, but rather a slight deviation from the marvelous Krypton 5 that could appeal to more people. 

When it comes to the bass response, Fir Audio is ruling the game and they’re just playing alone on the playground. No other IEM can come close to any of the Frontiers series IEMs. When it comes to a comparison of the bass response between the two, I’d say that the Radon 6 offers a thicker note, while the Krypton 5 has better slam and attack, sounding more vigorous and dynamic. Which works better will highly depend on the music you’re currently listening to or pairing with different gear. 


The Dita Perpetua is priced similarly ($2999) to the Radon 6, so this comparison is needed as well. 

The Perpetua is definitely a more laid-back sounding of the two. This is the IEM that reminds me of classic BBC speakers, with a sound that is so easy to listen to, yet extremely pleasing for long listening sessions.

Starting with the bass though, the Perpetua is not even close to the Radon 6, which offers a better extension, better physicality, slam, and raw power. There’s not even the slightest competition here, Kinetic Bass is just far superior to the Dynamic Driver found in the Perpetua.

When it comes to the midrange, the Perpetua is the more recessed sounding of the two, with lesser weight and less warmth. However, it’s also smoother and more intimate sounding, with its beautiful effortless type of presentation. The Radon 6 on the other hand sounds more dynamic, lively, and much more forward, with more body and presence.

The treble is definitely more detailed with the Radon 6, but it’s more easygoing in the Perpetua. High frequencies continue the trend of a sound that is just meant for chilling when speaking of Perpetua, while the Radon 6 is more expressive and has better sparkle, while not being harsh at all. When it comes to technical aspects, the Radon 6 is much better here, with better detail, resolution, and dynamics. 

A similar story with the soundstage, where the Radon 6 is wider, deeper, and offers more sublime imaging. The Perpetua is intimate in comparison, but it doesn’t offer the level of technical superiority that the Rn6 does.

Now let’s compare the Radon 6 to the flagship of the Frontiers series, the Xenon 6. 
The Rn6 sits in between the Krypton 5 and Xenon 6 when it comes to tuning. It’s definitely warmer and thicker sounding than the Kr5, but not as much as the Xe6. The Fir Audio flagship offers even more warmth, lushness, and richness to the sound, but it might actually be too much for many people. The Radon 6 takes those aspects and goes a bit easy on them, creating a sound that is going to be more oriented towards neutrality, but not quite there.
The soundstage capabilities of the Radon 6 are more impressive than that of the Xe6, with increased depth and better layering. The Xenon 6 sounds a bit more closed-off of the two, while the Radon 6 resonates fully with a vast amount of air around you. 
Also, the bass response of the Rn6 is more dynamic and crispier, while the Xe6 is even more prominent and heavier sounding. This is going to come down to your subjective test, whether you’d like more impact or more body. Both IEMs are absolutely spectacular when it comes to bass response, so…choose your weapon.
The biggest difference between the two is that the Xenon 6 is a darker-sounding IEM in general. Because of that, it might sound less detailed and sparkly of the two. However, this kind of tuning definitely has its fans, and I actually enjoy the XE6 for its incredibly rich and cozy sound very often.

HiFiMAN Svanar

The Svanar is a current flagship of HiFiMAN when it comes to IEMs. This is a very impressive IEM that is both technical and enjoyable, with great dynamics and a fantastic soundstage.

The Radon 6 is definitely fuller-sounding, warmer, and more bassy of the two. The bass of the Svanar, while very impressive, isn’t really a match for the Kinetic Bass technology, both in terms of physicality and rumble.

The rest of the spectrum is definitely more neutral and flat-tuned in the Svanar, resulting in a more universal, natural tuning. The Radon 6 however has that richness that the Svanar sometimes lacks. When it comes to the technical performance, I’d say that the Radon 6 is slightly superior here, but it’s not a level of difference you should really be worried about.

Both IEMs are fantastic when it comes to soundstage, and I cannot choose one over the other. The Radon 6 has more convincing imaging, but thanks to the superior speed of the Svanar, the amount of air and separation you’re getting is just incredible. The soundstage is probably the best aspect of the Svanar, so it’s definitely not an easy opponent.

When talking about the sound as a whole, I find the Radon 6 to be more interesting. It has that timbre and richness that make the music more enjoyable for me, while the Svanar focuses more on the neutral and technical type of approach. At the same time, the HiFiMAN flagship is significantly more affordable, but the build quality is miles behind. For that, I believe it’s worth paying more for the Radon 6 – you’re getting a WAY better build quality and design, and the sound that is just more sublime, enjoyable and unique.

Fir Audio Radon 6 Review – summary

Fir Audio once again released an IEM that is a force to be reckoned with. The Radon 6 sith in between their hugely popular Krypton 5 and Xenon 6. Not just in terms of the price, but the tuning as well.

This is a warm-ish sounding, thick and engaging IEM that doesn’t compromise technical performance. It comes with the worlds-best bass technology, incredible soundstage, and design/build quality that is simply the best on the market. While I personally still prefer the Krypton 5, I’d like to remind you that the KR5 is my favorite IEM of all time. 

If you’re looking for a high-end IEM that does it all, has interchangeable modules to alter the sound and that is just a piece of art in every single way possible, buy the Radon 6 while you still can.

Highly recommended!

Big thanks to Fir Audio for providing the Radon 6 for this review. I wasn’t paid or asked to say anything good or bad about this product, all of the above is just my personal, unbiased opinion. Fir Audio hasn’t seen this review before publishing it.