Testing Methodology: Measured using Arta via IEC 711 coupler to Startech external sound card. 7-9KHz peaks may be artefacts/emphasised due to my measurement setup, less so with deep fit. Measurements besides channel balance are volume matched at 1KHz. Fit depth normalised to my best abilities to reduce coupler resonance. Still, due to these factors, my measurements may not accurately reflect the earphone or measurements taken by others. I gave the M4 100hrs of burn-in to ensure maximum performance prior to subjective breakdown.
Fir reason that the M-series are more prosumer orientated and, with exception of the M2, all follow a similar kind of curve. The VxV sits apart with less warmth, though the M4 is likely its closest neighbour for those craving an upgrade. I don’t see huge inspiration from any reference curve here, perhaps you could loosely consider them a warm Harman derivative. The M4 has a slightly brighter tilt than the M3 and M5 with a more prominent lower-treble and less upper-bass hump. In turn, the M4 comes across as the cleanest and most precise of the bunch.
It retains a deeply-extending and powerful bass response followed by a clear yet naturally voiced midrange. However, it does have a slight bright tilt due to its more forward treble. What elevates this above your usual lower-treble emphasized in-ear is that it executes its treble forwardness in a progressive manner and has huge technical ability working in tandem to display notes with an incisive quality. As there are no isolated peaks, it doesn’t come across as overly aggressive and sharp, but defined, airy and energetic. The M4 radiates high-end IEM energy with exemplary detail retrieval augmented by a sound signature that brings its strengths to the fore.
I greatly enjoy the bass performance on Fir’s monitors for their intriguing physicality and awesome driver control. The M4 perhaps exemplifies this best as it has the cleanest tonality. In turn, you are best able to appreciate its technical chops. The tuning is progressive, with a deep-bass emphasis extending partially into the mid-bass before a progressive slope bass into a recessed lower-midrange – laying the foundation for a well-contrasted and energetic presentation. Sub-bass extension is excellent and a clear step up from the VxV but it also isn’t too emphasized at the very bottom unlike many competitors. This permits a more separated mid-bass and prevents over-pressurisation that can fatigue the ear over time – likely ATOM is aiding this impression too. There’s a light pressure and hugely satisfying slam and punch properties granting lows a sense of grandeur even without huge emphasis. The timbre remains mostly natural despite this; the note structure is simply bolder and more weighted than neutral forming a muscular over textbook linear presentation.
What separates the M4 from a lower-end hybrid is the perspicacity of its note presentation. The M4 has awesome driver control and the overall tightness of its bass response works hand in hand with its tuning to endow an addictive rumble and tactility. These qualities have likely been achieved by leveraging the mass of the housing, as Fir have been able to achieve an especially fast decay for a dynamic driver and a concise attack too. Accordingly, it doesn’t have huge, warm and lingering notes. The emphasis here is on lightning fast pace alongside outstanding separation and note definition that allow the M4 to more than keep up with even the most complex tracks. This is all the while upholding excellent dynamics due to the slightly more robust tuning in tandem with its strong extension. The speed of decay is unorthodox, but the presentation remains undeniably DD in terms of its texture. The combination of tactility and texture makes this one of my favourite bass responses from a hybrid earphone yet.
The midrange on the M4 also has a less orthodox tuning coming from the stringent reference-curve adherence we’ve been observing in lower price range. Nevertheless, the net result achieves a happy middle-ground between timbral accuracy for professional use and musical enjoyment for enthusiasts. This is a high contrast, revealing sound stemming from the lower-mid nadir and subsequent rise to 2kHz prominence. I applaud Fir for the progressive nature of the tuning; while it surely lacks the linearity of a pure reference monitor, the voicing isn’t far off, nor does it make many concessions relative to one either. It does so whilst balancing a more engaging low-end and treble alongside greater clarity in the midrange itself, crafting a more consumer palatable and equally genre-flexible musical representation. Though clear, vocals remain well-sized due to the decision to emphasize the 1-2kHz region over higher frequencies. They achieve appropriate balance with instruments, themselves empowered by the more robust bass tuning. The uptick of bass presence imbues a light warmth throughout, and a pleasant sense of body that helps to balance out the M4’s more revealing lower-midrange tuning.
For the purists out there, this is where the timbre deviates most as the note body remains lean albeit, not to the extent of dryness. The M4’s midrange is thereby characterised most by its high textural content with very high note definition and enhanced separation. It never sounds shouty or strained in its pursuit of clarity and the overall note presentation retains admirable coherence on account of the progressive fall off through the upper-midrange. Impetus lies on the progressive nature of the tuning with no dips to harm openness nor extension, both of which operate on a high level. With a hint of emphasis around 5kHz, the monitor has a slightly more articulate and delicate quality further enhancing note definition. It is a highly resolving sound and again, isn’t pushed to the realm of sibilance or sharpness due to the small deviation – though a slight breathiness can be heard on modern pop especially. Overall, convincing note density and smoothness paired with high definition contributes to a great sense of refinement. It is impressive that Fir have managed to craft a presentation that is so resolving yet simultaneously lacking in odd timbral or positioning characteristics.
It’s also intriguing to see how Fir have approached the top-end tuning on the M4 in addition to the technical accomplishments on display. Similar to many high-end IEMs, you get excellent resolution and detail retrieval enhanced by an crisp and energetic presentation. In fact, resolving power here is some of the best I’ve heard. Fir have approached this using a modest 5kHz bump that grants a more defined leading edge to notes, working in tandem with an incredibly clean transient response. As note attack is also sharp and highly resolved, it does have quite a lot of crunch so some aggression does creep into its detail presentation, lending the impression of brightness. I didn’t find it to be fatiguing nor sharp and tip choice can help, but be warned the treble does lead in the presentation. Where the M4 differentiates itself is with its more linear middle-treble region that gives it air and headroom well beyond the majority. Perhaps this can be considered rather a wideband boost given that this region is rather attenuated on the vast majority of IEMs. As the lower-treble hasn’t been over saturated, Fir was able to accomplish this without over-brightening the presentation – it is focused and vibrant, but not fatiguing nor sharp.
This is topped off with terrific top-end extension and a moderate bump in upper-treble energy. The net result is exemplary foreground detail retrieval with minutiae on full display and each note effortlessly discerned on even the most complex tracks. Separation especially is enhanced by the monitor’s thinner note body instigated by the lower-treble lift and exacerbated to some extent by the upper-treble tuning. Together, they create a delicate and dainty over smooth and organic note presentation. However, as the middle-treble isn’t attenuated as on many competitors, you do get a more natural shimmer and instrument decay that contributes to a textured image nonetheless. The tuning and extension on display work to provide extreme resolving power of background and micro-details. They remain presented in a relatively authentic manner relative to many monitors with more overt treble emphasis. While it is on the vibrant and energetic side, the level of resolving power and refinement is truly some of the best one can experience.
With strong treble extension and resolution comes a very impressive ability to image and immerse. The M4 provides width well beyond the head and its powerful bass services almost equally impressive depth. The M4 is able to position across a spectrum along both the lateral and coronal axis. Combined with its lightning fast transient response, sharp directional cues and excellent micro-detail retrieval, it has an especially holographic and multi-dimensional presentation that is very immersive. That said, the reduced contrast between foreground and background do also mean it doesn’t separate layers as well as some. In addition, it has less focused localization, favoring space, speed and detail presentation at the cost of being slightly floatier and more diffuse. Separation is excellent throughout, being sharpened just the right amount in just the right areas to enhance note definition and delineation without overshadowing neighboring elements. This is another factor contributing to the M4’s huge resolving power, practically highlighting fine details to the listener.
The M4 has unspecified sensitivity paired with a very low 6.4-ohm impedance. Altogether a very efficient earphone that doesn’t require heaps of voltage to reach high listening volumes.
Output Impedance Sensitivity
I have heard some mention that the M4 has a linear impedance curve, but this does not appear to be the case in my testing. Switching from the Shaling M2X (1-ohm) to the Hiby R6 (10-ohms) revealed a pronounced shift in the frequency response, with a blunting of the high-end and more prominent, woollier bass. It was listenable, not a huge change so it may be reasonable to experiment with 2-ohm sources like the Astell & Kern Dual DAC Cable if you are sensitive to high frequencies. If you’d like the enjoy the M4 as the manufacturer intended, you will have the best experience listening from sources with a 1-ohm and below output impedance. I also did not hear huge changes switching to the Topping A30 Pro with its 0.1-ohm impedance relative to my THX789 with 1-ohm impedance.
The M4 is a very efficient earphone and doesn’t require a heap of driving power in turn. However, as it is resolving, you will want a higher end source to experience the best imaging performance and resolution here. Switching to my THX789 from the M2X revealed a slightly more controlled and defined bass. However, it was clean and balanced from both sources. The same goes for the midrange, the treble was a little crisper and more defined on the THX789 but it also tends to be a sharper sounding amplifier. The biggest difference was the soundstage depth, which was much more two-dimensional on the M2X. The THX provided a more resolving treble with greater projection and better extension. On volume 0, hiss was clearly audible on the M2X and was just inaudible at low listening volumes. I will pick up source noise more than most but still not nearly to the extent of something like CFA’s flagships, so it is not a notable flaw of the design to me.
Suggested Pair Ups
The M4 is bright, but it is not to be mistaken for a light-footed earphone, having more than a fair amount of bass oomph to it too. This means it isn’t all that sensitive to source coloration as some. The biggest consideration is the output impedance of the source as the top-end is rapidly attenuated with higher impedances. This can be used to your benefit if you prefer a smoother sound, but do consider this if you don’t. Driving power is not so much of a concern, you always receive a controlled and dynamic bass, but resolution and imaging is very important, with huge ability to scale. In terms of colouration, I did find myself enjoying the more linear sound of the Topping 30 Pro stack as my THX789 was a bit over-sharpened in the lower-treble, losing some texture. I would avoid analytical sources personally but if you are a detail head, there is plenty to be enjoyed on these pairings.