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FiR Audio’s House of M: The M4 – An In-Ear Monitor Review

DISCLAIMER: FiR Audio and Project Perfection provided me with the M3, M4 and M5 in return for my honest opinion. I am not personally affiliated with these companies in any way, nor do I receive any monetary rewards for a positive evaluation. I’d like to thank FiR Audio and Project Perfection for their kindness and support. The review is as follows.

Next up in the M line, we have the M4. Also sporting white logos on either side, FiR’s universal quad-driver swaps out the M3’s greyish-blue faceplates for a clean, champagne colour scheme, which is my personal favourite of the three. Again, it also features this line-up’s stunning, machined-aluminium shells, as well as FiR’s crop of proprietary technologies. It adds an additional midrange driver over the M3, resulting in a meatier, more mature sound with improved technique to boot.

FiR Audio M4

  • Driver count: Three balanced-armature drivers and one dynamic driver
  • Impedance: 6.4Ω @ 1kHz
  • Sensitivity: N/A
  • Key feature(s) (if any): Direct Bore drivers, Tactile Bass technology, ATOM pressure release system
  • Available form factor(s): Universal aluminium and custom acrylic IEMs
  • Price: $1899
  • Website:

This review is a part of FiR Audio’s House of M series and will only cover sound impressions. For the introductory article covering packaging, accessories, build and fit, as well as FiR Audio’s slew of proprietary technologies, click the link here.


This M4 is where I think FiR’s line-up really picks-up technically, and where it establishes a clear house sound as well. Like the M3, it’s neutral-leaning with added presence in the high-mids and highs for energy, vibrance and contrast. Where the M4 differs is in its approach to the tuning. A slightly-less exaggerated, more refined low-treble peak removes some of the dryness the M3 could, at times, have. More crucially, the M4 comes with more body, wetness and warmth to its bass and low-mids too. The result is a more coherent, more natural-sounding tonal profile. It still places quite a bit of emphasis on cut. The shakers and plosives on Tori Kelly’s Kid I Used to Know sit just above all else, for example. But, it’s a more refined, less-coloured take on this sig that does the lower, warmer notes right too, serving them with texture, solidity and weight.

Further adding to that is this M4’s much-improved dynamic range and resolution, especially further down the frequency chart. Instruments come off better-rounded and three-dimensional, and you’re able to discern much more texture along the lower-mids too. They aren’t as scooped, so the elements I mentioned on the M3 section are allowed their weight and punch back. It does wonders in lending toms, trombones and male vocals power, for example. But, as crucially, it doesn’t ever congest the image either. The M4, thanks to its cut, is still capable of near-surgical separation, and I think it achieves a good balance between sounding big (or immersive) and sounding tight; organised. Coming full circle, the M4’s high-end extension is a gift to its dynamic range. Instruments are allowed to breathe more, and, when they punch, they do so with more oomph. They ebb between loud and quiet more palpably than on the M3, for a tonality that simply feels more alive.


With the M4, FiR have beefed up their DD-driven low-end, giving it a fuller, gutsier presence in the mix. Like the M3, it has immense power, physicality and slam that’s unmistakably dynamic. And, it sports a similar frequency curve as well. Most of its focus is in the sub-bass, which lends an almost-subterranean oomph to bass lines, as well as a thick, guttural thump to kick drums. It then tapers off as it approaches its mid- and upper-bass, which ensures a clean, musk-free canvas for its mids and highs to occupy. The one, big difference it has with that M3’s bottom-end is a few more dB’s of presence, which is the reason behind its more prominent role. In forwardness, it can creep up on the lead on certain tracks. Anika Nilles’s kick on Dark Chocolate, for example, slightly inches above the synths on the intro. But, again, because of how it’s tuned, it never detracts. Instead, it fills out the bottom nicely, contrasts against the treble and adds a good, ole dose of punch too.

That deliberate sub-bass tilt also decides the low-end’s final tonality. To me, it’s a darker bass geared more towards EDM or pop, which prefer guttural, concentrated slams over rich, warm blooms. Though it’s also a low-end that can work with soul or jazz, it does carry a certain colour with it. Listening to an upright bass like Bridgett Kearney’s on Lake Street Dive’s Baby Don’t Leave Me Alone With My Thoughts, you’ll tend to hear more of the thump coming from the string plucks, instead of the warm, woody resonance coming from the body of the bass. So, it isn’t perhaps ideal if you wanted a more organic, live-feeling sound. But, if your genres of choice were more “modern” anyway, then it won’t be a problem. Plus, the added bass presence should be enough to compensate too. In terms of extension and texture, the M4 – like its siblings – excels. The result is a tactile, dynamic, yet quick and unintrusive low-end, that is as three-dimensional as it is a treat to listen to.


The M4’s midrange follows a fairly similar curve to the M3’s; laid-back in the lower registers, followed by a healthy 1-2kHz rise that’s maintained into the upper-mids. Like its little brother, then, vocals on the M4 tend to be on the tighter, cleaner side with wonderful separation, clarity and headroom. However, unlike the M3, the M4 doesn’t have that slightly-artificial weightlessness I heard on the former, thanks to its lifted low-end. The added higher-bass content has especially rescued baritones like Michael Bublé, who, on I’m Your Man, showcases tons more depth, power and gravitas. His voice’s still ever-so-slightly on the tighter side; not dripping in butteriness or husk. But, it’s a tonal boost over the M3 nonetheless, and it’s one that renders this M4 much better-rounded as well. Violas sound just as dynamic as violins, snare drums crackle and thump at any pitch, and horn sections are far more balanced too. Overall, it’s a linear tone that hides its colour very well.

Also like the M3, this M4 sports excellent texture, resolution and crunch. Lead instruments – whether it be vocals, pianos or electric guitars – stand out with immense power, and they’re etched very precisely too. No matter the genre, that lead is guaranteed to project right through the mix, but without any honky-ness or shouty-ness to them either. It’s a hallmark trait between all three models, which I think serves all kinds of music equally well (as long as you like your vocals vibrant and clear). Over the M3, what I think this M4’s extra driver gives its midrange is authority and depth. Vocals ebb and flow naturally without restraint, and their power is accompanied by an effortlessness I felt the M3 didn’t yet have. This, to me, made it easier to pick out little shifts and nuances like Robbie Williams’ playfulness I mentioned on the M3 review. And, it also allowed vocals to almost-singlehandedly carry entire songs, which is crucial for barer arrangements like Lucy Rose’s Floral Dresses. While I felt the M3 could only contribute the clarity and vibrance of its mids to a bigger picture, the M4 has really stepped it up here for a midrange that’s a star all on its own; clearly-hued, richly-nuanced and holographic as well.


Up top, the M4 continues to stay faultless for the most part, but it does end up indulging in a bit of colour around its low-treble. The IEM employs a 5kHz peak that nearly stands out as the loudest sound in the mix, which adds a notably bright, crisp edge to its attacks. Percussive sounds like snares, cymbals and string plucks especially benefit from the colouration as they’re gifted wonderful clarity and impact. Cymbals and chimes on the periphery of Dirty Loops’ Work Sh*t Out sound pristine, for example. Vocalists articulate very cleanly because of it too. But, there can be points where the rise comes off excessive. Almost every s on the live version of Adele’s Daydreamer tizz’d and buzz’d. The same went for Sam Smith’s One Day at a Time. But, again, it’s not a fatal flaw, so much as it is a situational quirk. It doesn’t harshen smoothly-made songs like Tom Misch’s South of the River. And, if push comes to shove, there’s always the option of tip-rolling. So, for me, I’d say to keep the peak in mind, but also to not let it spook you from giving it a go and seeing how well it agrees with your ears.

Following the lower-treble peak, you have more of that natural, airy and linear high-treble that I loved on the M3. Again, I must commend FiR’s restraint at least 7kHz and up. They’ve given the region the right amount of presence relative to the mids and lows without over-aerating or overexposing the image. And, to me, this does wonders particularly for vocalists and percussion. The former have a fine balance between that warm roundedness and the more articulate, throaty notes. The same goes for the latter, where toms and lower-tuned snares have the same fullness and punch as the cymbals, and the kit doesn’t sound too airy or hollow, as if the overhead and room mics were amplified. With that tonality comes great extension and resolution too; greater than the M3’s. Cymbals and hi-hats feel better-defined with stronger texturing and punchier hits; again, without coming off bright. And, the M4 images with superb authority too; a wide stereo spread, pin-point placements and a clean, stable backdrop right below. FiR closes out this M4’s treble with detail and finesse to boot.

General Recommendations

While the M3 was made for those after a crisp, articulate sound with licks of warmth, the M4 is almost the inverse; an in-ear with a largely-linear, pleasing, versatile timbre, topped with touches of crispness, air and sizzle up high. As you might assume, then, there isn’t really a genre the M4 won’t pair agreeably with. It has both the midrange quality to carry singer-songwriter genres and the contrast necessary for louder, more explosive arrangements. The technical improvements it’s received over the M3 give it the headroom, imaging and resolution needed for more complex mixes too. The only caveat I’d note towards this M4 is, again, its low-treble peak. If you’re sensitive to 5kHz peaks, which tend to add sizzle to s notes and hi-hats with certain recordings, then you may want to be wary of the M4 with the stock, silicone tips. Otherwise, FiR’s M4 is a strong all-rounder that leans towards articulation and clarity, but remains largely, impressively colourless as well.

Select Comparisons

Empire Ears Phantom (USD 1799)

Compared to this M4, Empire Ears’ Phantom is a fuller, more saccharine-sounding monitor with a significantly richer low-midrange. Instruments are bigger in size, they’re more saturated in tone and they’re a bit darker-sounding too. Whereas, the M4’s presentation is comparatively drier, crisper and airier, but still within the realm of balanced or natural. What the two have in common is a linear mid-to-upper-treble that doesn’t overstep the mids. Vocals and percussion are rarely dry or hollow-sounding on either monitor. But, the Phantom does have the more drastic treble shelf of the two, while the M4 keeps quite a bit more top-end energy for openness and air. Down low, you’ll get a ton more fat out of the Phantom, due to its richer lows and low-mids. But, the M4’s DD keeps up in the sub-bass, so it has punch too without the extra warmth.

Spatially, there isn’t much to separate the two in terms of raw size. But, you’ll get an airier, more free-sounding stage out of this M4 due to its – again – more neutral tuning. The Phantom’s meatier instruments fill up more of its real estate, and its shelved high-treble makes way for warmth to lightly permeate that as well. Instruments are positioned more precisely and separated more cleanly on the M4, while the Phantom’s bigger notes can be a hair more susceptible to overlap in its space. The former’s instruments are also more tactile and physical-sounding, so it could be considered more resolving in that way. But, the Phantom does supersede it in terms of discerning the different colours or hues within a track, and not giving everything a similar coat of paint, so to speak. At the same time, some may not want that aspect and prefer a safe, versatile sig. So, take the M4 for clarity and air with balance in tone or Phantom for a warm, saturated, timbre-led sound.

Empire Ears Valkyrie (USD 1599)

The Valkyrie and M4, despite their fairly-differed driver configs, have quite a few qualities in common. Both have punchy, dynamic signatures with body stemming from the mid-bass, followed by a hearty 1-2kHz rise, and finished with a refined upper-treble that’s never off-rein. To me, the differences start at the extremes. Down low, the Valkyrie is fuller and more present. Kick drums have quite a bit more meat and warmth to them. Though the M4 has good punch to it too, it doesn’t go quite as all-in as this Valkyrie. A quicker decay makes those lows a bit drier by comparison too. Both differences mean the M4 has the slightly more precise bass and the cleaner stage, while the Valkyrie edges it on fun. Up high, the M4 has a sharper, more present low-treble than this Valkyrie, which lends its transients – and instruments like snares and cymbals – a crisper bite. Which one you’ll prefer will depend on your tastes, but this does give the M4 a lead in picking out details.

With that said, the two aren’t far off in raw resolution. I find this M4 still has that edge in texture and tactility. It’s capable of fleshing out instruments and presenting them with a hair more rawness and physicality, and that’s most prevalent for the bass and treble. In the midrange, it’s more of a wash between these two. This Valkyrie’s stronger 2-3kHz region gives female and higher-pitched male vocalists a rounder tone; say, Tori Kelly and Cody Fry. But, the in-ear’s less present, drier low-mids make it less capable with lower lead tones than the M4. Instruments like trombones and baritones like Michael Bublé will show more weight and authority on the latter. Dynamically, Empire’s Valkyrie brings a more realistic, palpable thump than the M4 down low. So, despite the M4’s clearer bass, I do feel the Valkyrie beats it in realism. The midrange is, again, fairly equal. And, up high, it’ll again come down to preference. This Valkyrie’s calmer low-treble will come off more controlled and refined, but some may prefer the M4’s greater bite and punch there. So, as always, your mileage will vary.

Stealth Sonics C9 Pro (USD 1499)

Going from FiR’s M4 to Stealth Sonics’ C9 Pro is the epitome of sonic whiplash. One dips where the other peaks and vice-versa, placing these two in decidedly different demographics. The C9 Pro is an exceedingly warm, lush and smooth IEM. I wouldn’t be surprised if trebleheads called it veiled. Whereas, this M4, though fuller-sounding than the M3, still has much more of an edge thanks to its low-treble peak. To me, however, the main differer lies further down the range; in the low-mids. While this M4 dips between 300-1000Hz to tighten instruments and provide headroom, the C9 Pro opts instead for a broad bump across the bottom-half of the spectrum, which is what gives it its wetness, richness and body. Keys, vocals and strings will have more of a pillowy, forgiving tone to them. Then, percussive elements like snare drums, cymbals and bass slaps will sound notably damped or diffuse relative to this M4 too, whose 5kHz peak really highlights those attacks.

Despite its warmer, less articulate tonality, though, the C9 Pro is an IEM that still sports admirable amounts of openness, precision and air. It spreads and positions instruments just as capably as this M4 does, though the latter’s crisper, tighter notes will give it that extra, pin-point quality that the C9 Pro doesn’t quite have. The Stealth Sonics flagship’s larger, fatter instruments can be prone to ever-so-slightly spilling into each other, while the M4 tidies its stage strictly. The former may be preferred for a more euphonic, free-flowing profile, while I’d recommend the latter to those who want utmost fidelity and precision. Spatially, it’s a fair wash in width and depth, though the M4 does show more height. It’s roomier, due to its tighter notes too. Where I feel this M4 really pulls away from the C9 Pro is in vividness and texture. There’s a tactility and physicality to instruments on FiR’s M4, which, with its punchier dynamics, allow them to sound more vibrant and life-like.





Church-boy by day and audio-obsessee by night, Daniel Lesmana’s world revolves around the rhythms and melodies we lovingly call: Music. When he’s not behind a console mixing live for a congregation of thousands, engineering records in a studio environment, or making noise behind a drum set, you’ll find him on his laptop analysing audio gear with fervor and glee. Now a specialist in custom IEMs, cables and full-sized headphones, he’s looking to bring his unique sensibilities - as both an enthusiast and a professional - into the reviewer’s space; a place where no man has gone before.


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