FiR Audio Radon 6 Review featured image

FIR Audio Radon 6 Review

In this post, we review the FIR Audio Radon 6, which is a limited-edition flagship Kinetic bass-equipped universal multi-driver in-ear monitor. It is priced at $3299 SRP.

Disclaimer: This sample was sent to us for our honest opinion. Headfonics is an independent website with no affiliate links or partnerships. We thank FIR Audio for their support.

Click here to learn more about FIR Audio products that we have previously featured on Headfonics.

Note, that this feature follows our latest scoring guidelines which you can find in more detail here.

FiR Audio Radon 6 Review featured image
FIR Audio Radon 6 Review
The FIR Audio Radon 6 is the most complete and natural-sounding monitor I have heard from the company to date. It works wonderfully well with almost everything, especially if you want an open midrange with excellent levels of detail.
Slide here to add your score on the gear!81 Votes
Neutral tuning with excellent resolution
Flexible filter system
Durable compact design
Limited availability
Tiny ATOM XS filters could get lost easily
Reader's Score

The FIR Audio Radon 6 is a new, albeit limited edition flagship universal in-ear monitor launched in the spring of this year. It is based on the current Frontier Series flagship the Xenon 6, but with some modifications including the sound signature. I did say limited so that means there are but 300 production units worldwide available.

I first heard the Radon 6 on day 1 at CanJam Singapore 2023 in March and since I had never heard the Frontier Series Xenon 6 on which it is based I made sure to hear them both at the same time. They do sound quite different!

At $3299, the Radon, or the Rn6 as its popularly referred to, is comfortably their flagship universal IEM offering while stocks last. 

From the website, it might seem that the Radon 6 stands alone in their lineup, rather FIR has positioned it separately due to its limited availability but it’s still part of the Frontier series out of which we have reviewed the Krypton 5 to date.

CanJam Singapore 2023

Tech Highlights

The FIR Audio Radon 6 is a universal hybrid multi-driver IEM. It is in universal format only currently but hopefully, there will be a custom version soon, like the core Frontier Series lineup.

On a high level, it consists of a tribrid mix of 6 drivers including dynamic, BA, and EST. The precise grouping is a single Kinetic bass 10mm driver for the lows, with 5 OpenDriver BAs including 2 for the mids, 2 for the mid-highs, and 1 for the highs using FIR Audio’s patented Sound Reflector technology. The final driver is a Sonion Electrostatic driver for the ultra-highs.

As with the Xenon 6 and Krypton 5, the Radon 6 uses tubeless armature designs which are denoted here by their OpenDriver nomenclature.

Kinetic Bass

The Radon 6 also uses FIR Audio Kenetic bass technology for the lows. The “Kinetic Bass Dynamic Driver” houses a hybrid bone conduction technology that delivers both bone conduction as well as traditional air conduction at the same time.

That should bring an additional dimension to how you hear and ‘feel’ the bass delivered by the Radon 6 with a mix of vibration and soundwave rather than just soundwave.

The 10mm dynamic driver is open and ported to the exterior of the shell though unlike the Krypton 5 design, there is no transparency to see its positioning internally. Rather, you have a big fat grill or port on the underside of the shell as a big giveaway.

This porting allows low-end notes to be transmitted directly to the ear cartilage and surrounding area in the form of kinetic energy that travels to the inner ear via bone conduction. Ideally, it opens up the bass performance beyond traditional driver responses producing a bigger and deeper tone and one you can almost ‘feel’.

Open Acoustics

As with the Krypton 5, the Radon 6 also uses a BA OpenDriver for the highs in combination with their Sound Reflector technology. 

A sound reflector is part of FIR Audio’s wider ‘OPEN ACOUSTICS” System whereby the tweeter is further forward in the internal driver arrangement being positioned right in the nozzle. The reflector then controls the high-frequency response and directs it into the ear canal in a smooth and clear manner.

The tubeless BA driver implementation, (OpenDrivers), is also another feature of the OPEN ACOUSTICS technology and it is something we have seen time with FIR Audio’s lineup before with the Krypton 5 but also on the more affordable Bellos Audio X2 staging capability

The Radon 6 OPEN ACOUSTICS design is further enhanced with another internal acoustical chamber which replaces more traditional dampers and filters in shaping the sound from the open drivers.

FiR Audio Radon 6 filters


This is the first time I have used the ATOM XS system in a FIR Audio review. Previous reviews are all about the larger ATOM X system though the principle and application behind them are much the same.

The key difference is the diminutive size and how they are exchanged which is a little more precise. So much so, FIR Audio has included a 9mm precision hex screwdriver alongside them as that is the only way you will successfully take them out or put them back in again.

For those that do not know, ATOM stands for Air Transferring Open Module and on a high level vents harmful air pressure out from a sealed ear canal. It is designed to offer a measure of protection from hearing damage as well as shape the final sound signature, for example, more bass or a wider soundstage.

The Radon 6 comes with a total of 4 ATOM XS filters; 3 in the tray and 1 in the unit out of the box. They are finished in various colors for quick identification and they do have an effect on the level of isolation with a range from 10dB up to 17dB as well as the perceived bass level and soundstage properties.

FiR Audio Radon 6 design


The Radon 6 is a compact but sturdy form factor with a recognizable design language that pulls from the Belonozhko brother’s time in 64 Audio through to earlier FIR Audio M-Series formats such as the M4

The aesthetics are opulent and classy and quite nicely blended together. You get a mix of a new satin black machined 2-piece aluminum shell with a creative and unique mix of gold flakes under a sapphire crystal glass face plate.

On the top, you have the familiar ‘bunny’ gold logo on one of the driver’s plates with the Rn6 moniker on the other in a similar gold finish.

To the rear, a fingerprint-free matte-finished shell is interrupted by a large circular grill with the Kinetic bass venting port sitting behind it. You also get a metal grill finish for the Radon 6 snap screen nozzle guard to prevent dirt buildup.

The Radon 6’s rigid 2-pin 0.78mm connector stems are a familiar perpendicular finish sitting relatively close to the top of each shell with the tiny Atom XS filter sockets sitting to the front of the stems. They are rated for over 1k connections so great for cable rolling.

The Radon 6 also takes advantage of FIR Audio’s Rigid Technologies which means a substantial portion of the design elements are easily serviced and should last a long time. This included the 2-pin connectors, the snap screen as well as a very durable shell.

The dimensions of the shells are relatively deep, stubby in a way but not too wide, and with a very smooth finish that is unlikely to grate your ears during use. Honestly, with this kind of build, they feel like little tanks and can probably take a fair beating during everyday use compared to resin formats.

FiR Audio Radon 6 comfort

Comfort & Isolation

The compact nature of the Radon 6 makes them a very comfortable fit. They are compact enough to stay well out of the way of any bumps and ridges in your ear’s canal and concha basin. 

The majority of the canal fit and how securely they sit in your ear will come from the supplied stock tips and there is a wide selection there supplied by FIR Audio. 

What you will notice is that the level of isolation will depend on the filter you use combined with that big venting port.

As such, and like any hybrid IEM, the isolation will be a step behind sealed BA monitor shells that do not offer any venting. On the plus side, with the various filters you can adjust the level of background noise you want to experience.

The filters themselves are color coded to help you quickly identify which one suits you the most. The pre-installed red or -0 dB filter will provide the lowest passive isolation capability.

You can close the gap with the yellow filter with an additional -7 dB of isolation but it won’t isolate quite like a custom version. I suspect the filters will be treated more like tuning modules in the long run but there is no denying that they do work really well in terms of lowering general listening fatigue from my previous experience with the Krypton 5. 


You get two types of tips with the Radon 6, a hybrid silicone and foam Symbios W in large, medium, and small, and 3 regular foam tips in similar sizing. 

Both sets of tips isolate equally as well though I find the stock foam tips a little more comfortable for long-term use compared to the stiffer finish of the Symbios tips. That being said, they will produce a slightly different sound signature with the Radon 6 and this is where I prefer the slightly more defined and cleaner or punchier tone of the Symbios W tips.

FiR Audio Radon 6 stock cable

Stock Cable

We have a new stock cable for the Radon 6 and one that is also made exclusively for this monitor. This time we have a balanced terminated 48″ 26AWG 8-wire geometry with pure silver and copper shielding as opposed to the 48″ 26AWG SPC inside the Frontier Series ‘Scorpion’ wire.

This is a relatively beefy-looking cable but with a slick-looking dark brown-toned braided PVC jacket and a 4.4mm gold-plated branded Pentaconn plug it behaves impeccably with very low microphonics and zero memory retention during use. 

It’s fairly light for its size also and likely down to the low-profile black metal splitter and chin cinch rather than anything bulky or grandiose as well as matching colored 2-pin connector barrels that beautifully blend with the Radon 6 black shells when connected.

What I especially like is the subtle additional memory coating for cable shaping around the ear. It is very low profile, almost invisible to the jacket itself so it has a nice soft springy feel and is very comfortable around the ear as a result.

FiR Audio Radon 6 unboxing

Packaging & Accessories

The Radon 6 packaging follows a similar look to our previously reviewed Krypton 5 compact black box and branded packaging which is professional looking but nothing terribly grandiose. Don’t fret, you still get a FIR Audio Space Force patch :)

The external design is very much in keeping with their company ‘bunny’ branding with a black cardboard box beneath using a simple lift lid design.

Inside, you get the Radon 6 monitors and the cable neatly displayed on top, with the case, manual, and accessories presented inside a deep-layered protective foam underneath.

The case is an elegant black with white stitching leather puck-style case with the FIR Audio branding on the detachable lid.

The case is a bit big to be called pocketable but there is plenty of space for cables, accessories, and drivers. The internal walls have a soft felt material also to prevent accidental damage from moving around. Despite its leathery finish, the frame and base are quite rigid so it is also quite durable for handling.

The carry case itself acts as a clever holder tray for all of the useful accessories including the tips, cleaning brush, and filters/screwdriver. And yes, there is enough room for the cable and monitor shells to fit in alongside the accessories tray.

Click on page 2 below for sound impressions and pairings.

Click on page 3 below for our selected comparisons.

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