Sound –
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.
Tonality –
The U6t provides to my ears, a U-shaped signature biased towards the bass but with an energetic treble that cuts through its thicker mix. Low-end emphasis is contained mostly to the deep-bass which is reminiscent of the Harman curve but serves as a slightly warmer take here. The small 5k peak above precedes a darker background. Between lies a natural midrange that I really cannot criticize in terms of raw tonality. The restraint exercised in its tuning also means the U6t isn’t an instantly gratifying listen, there isn’t huge upper-treble sparkle nor the most palpable sub-bass pressure. Much like the original, the U6t’s call to fame is its gorgeous and refined tonality, especially in the midrange, paired with elevated headroom but not excessive brightness in the treble. Compared to that model, the tuning showcases a clear step forward in timbre and balance whilst retaining a core character derived from tasteful organic warmth and a well-structured, coherent note presentation.
apex Modules
As aforementioned, the U6t comes with 3 apex modules. These essentially alter how open the shell design is. As openness increases so does the perception of the soundstage. Conversely, efficiency decreases, and bass diminishes in addition to passive noise isolation reducing. Do note, however, that the perception of sound changes with a more open form factor. This means the effects of the modules in listening will alter from their effect on the raw frequency response; and it cannot be considered as linear a change as something like an interchangeable nozzle filter. Previously, I’ve always had the impression that 64 Audio used the various apex modules as an extra instrument to tune their earphones and you would usually find the stock modules to sound best by a fair margin. However, here it feels like a genuine tool to tune to listener preference, all being viable options to suit different preferences. Do note that for the sake of review, I will use the stock M15 modules that provided a balanced mix of qualities to my ears.
MX Modules
Where on some of 64’s models I find the MX modules to be overly lean, in the context of the U6t’s bassier tuning, they work to good effect. The black MX modules offer -10dB of isolation and are the most open currently available. As measured, you get a considerable drop off in bass presence in addition to a slight forwardness in the midrange, and minimal changes to treble. As above, the shift isn’t quite as large in listening. What I’m hearing is a relatively flat sound signature that retains a slightly more robust, warm leaning bass voicing if not a forward bass response.
Weight and extension remains, and the midrange is natural and slightly more forward relative to the bass but of similar character. The treble retains a similar presentation as well, though I did note that notes didn’t have as much initial bite so these modules didn’t push the sound too bright or thin, retaining balance. Most noticeable, besides the bass attenuation is the perception of soundstage width which extends considerably wider than on the M15 modules. This is accompanied with reduced isolation and an increased sense of openness during wear. I find the MX modules to be a very viable listening experience.
M20
The M20 modules offer the inverse experience, being more closed and bolstering passive noise isolation to -20dB. You get a slightly more pressurised, stronger sub-bass kick and bass comes forward a touch too. I think bass lovers will appreciate this module especially as it does give you a more extended and slightly more aggressively textured bass response that is all the more impressive for a BA system. Above, mids are relatively unchanged but do sound slightly more laid-back relative to the enlarged bass. Treble sounds slightly more aggressive but not overly so. The voicing remains similar to stock and technical performance is also unchanged. The soundstage is slightly more intimate, width especially takes a hit. The imaging overall is less multi-dimensional compared to the M15 modules.
Bass –
If you’re a bass lover, the U6t provides power, rumble and impact in spades. It also does so in a rather innocuous manner that is highly enjoyable and avoids encroaching excessively upon the midrange. Emphasis is focused in the sub-bass and tapers off quickly above to retain cleanliness and separation. As the lift is quite large and extends partway into the mid-bass, bass is full and medium warm, though I wouldn’t say either define this earphone. Rather, the U6t is characterized more by its slam, rumble and elevated note weight. Its thick, emboldened notes hit hard, and extension is immensely impressive for a BA earphone if still shy of the visceral pressure of a good DD implementation.
I would agree in totality with my colleague Daniel’s review of the custom variant; the response here is biased towards dynamics over articulacy but this is, as with most things, a listening preference rather than a pro or con. Scrutinizing the U6t’s note presentation further reveals a pleasing mix of qualities. Control is excellent as you’d expect from a BA setup, you get well-defined notes and greater separation than a DD setup of similar tuning. Attack isn’t quite lightening quick and concise, prioritizing weight and impact over absolute speed and tightness. However, these do remain on the higher side, decay especially is quite fast, redeeming separation in context of its elevated note size and thick, meaty sub-bass emphasis. These qualities portend towards a bass presentation that doesn’t feel perfectly balanced but remains mostly clean and entirely fun.
Mids –
As the bass and treble have been brought forwards, the midrange appears slightly laid-back by comparison. Where the original U6 was also smooth and laid-back side, the new model achieves this style of presentation in a far more refined manner with less impact on timbre and separation. For the U6t upholds excellent linearity, showcasing a well-positioned lower-midrange and progressive climb to 2.5kHz prominence before a gently fall off in the upper-midrange. As its transition aren’t overly contrasted, colouration does creep in due to elevations in the bass and treble. However, the net result leaves a highly natural voicing within the midrange itself. Specifically, the U6t has a coherent, full-bodied and well-structured note presentation alongside a warmer tonality instigated by its low-end lift. Clarity is retained with its pleasantly articulate lower-treble that imbues additional note definition and just a little gloss but never any sibilance, sharpness or breathiness.
It is here that the U6t harkens back mostly to its progenitor, boasting superb coherence and just enough top-end lift to retain openness. Vocals are a little laid-back but well-sized and naturally voiced, the uptick of density aiding complete and fleshed out notes. It’s an enriched, creamy midrange with no huge fallibilities that will appeal to lovers of a more organic sound. As always, the repercussion of this style of tuning is a loss in separation and clarity, neither of which are lacking, but certainly aren’t outstanding either. The U6t doesn’t have the most revealing midrange presentation nor does it highlight layers and textural nuances. But I did find its more articulate nature to draw focus more to smaller details that such a tuning would otherwise, retaining a relatively complex image that is all the more impressive given its rich tonality. While you shouldn’t expect huge clarity, the U6t provides a highly enjoyable midrange balancing warmth, coherence and resolving power with aplomb.
Highs –
tia drivers are a wonderful thing in theory, and 64 Audio’s implementation perfectly captures its strengths and without much dilution here coming from their flagships. The U6t provides a well-positioned treble response that has a slightly vibrant, energetic voicing without skewing overly forward or bright. It sits just in front of the midrange and just behind the bass; so though crisp and clear, I never found treble to overwhelm. The benefit of the tia tech comes in the form of its pristinely clean transient response. This does, in part, draw parallels to the speed of the EST tweeter systems we’ve seen popularized recently. However, the difference here is the definition of the tia driver’s notes and the ease of delivering a more desirable frequency response.
Notes attack with surgical precision and decay with well-metered accuracy evoking a masterful display of fine details. Despite this, notes aren’t wispy or delicate but well-realised with abundant texture and none of the brittleness you may observe on many traditional BA setups. The tuning aids this impression, with peaks at 5 and 7kHz in addition to a lift in the upper-treble that contributes most to its more vibrant expression in the absence of overt audible brightness. As the 6kHz region isn’t boosted, you get a slightly detail-forward presentation without over-sharpening of the image. Accordingly, enjoyable note body and even a hint of euphonic warmth are retained, with the hyper-defined note presentation doing the heavy lifting from a detail retrieval point of view.
Above, the U6t does provide a little additional air over most IEMs and this contributes to excellent headroom if not a presentation that I would call explicitly open. Similarly, extension is good and you can perceive an upper-harmonic lift in the manner that the U6t expresses its treble instrumentation. However, sparkle isn’t especially overt here as you may experience on 64Audio’s higher-end models and even some competitors that shift the peak more into the audible range. This monitor focuses more on top-level foreground detail retrieval alongside strong background detail retrieval, whilst keeping its image clean and free of grain. The superb separating and resolving ability of its treble serves as a fitting foil to its bass and midrange while the cleanliness of its note delivery and background uphold a consistent overall character.
Soundstage –
I’ve usually been impressed by the staging on 64 Audio’s IEMs, especially with their slightly more open form factor courtesy of apex. The U6t diverges slightly with a medium sized stage that is far from the largest in-class but delivers ample dimensions to avoid congestion even with its thicker tuning – of course, this can be expanded with the MX modules. The upside is that its tuning produces a very well-rounded image, equally proportioned between width and depth. Combined with its imaging chops, you get an immersive multi-dimensional image if never a holographic one. This is not for lack of speed but is rather due to the darker, cleaner background and thicker note presentation.
Another highlight is its especially well-delineated layers with a focused foreground, excellent centre image stability and laid-back background. At the same time, it doesn’t discern between multiple layers, for instance complex vocal harmonisations, quite as well as IEMs with a more neutral note size. Localisation is also sharp, delivering pinpoint precision. Where the U6t falters most is with regards to separation which is to be expected given its tuning. To reiterate, it is not a bad performer in the slightest, but does fall behind many competitors. While it is an organized and composed performer, never sounding busy, the U6t possesses just enough space to encapsulate its thick notes, but there isn’t much space or air surrounding.
Driveability –
64Audio quote a high 108dB sensitivity and low 10ohm impedance on their website, making the U6t a very efficient earphone. Despite this, with their Linear Impedance Design (LID), listeners also shouldn’t have to worry much about source pairings from an output impedance point of view. This is a huge asset for those with a variety of players and audio professionals who require a consistent sound from multiple sources.
Output Impedance Sensitivity
Switching between the Shanling M2X (1-ohm) and Hiby R6 (10-ohms) revealed minimal shift in tonality beyond colouration of the sources themselves. This is congruent with their linear impedance design which means the user can pair the U6t with high output impedance sources to achieve their desired synergy without worrying about overly skewing the signature.
Driving Power
Switching from the M2X over to my THX789 + SMSL SU-9 desktop stack revealed that the U6t is an efficient monitor that doesn’t require much power to achieve a strong listening experience. The desktop stack did provide a slightly more extended bass in the form of a more affirmative and palpable slam at the very bottom. However, I didn’t feel bass was diminished overall on the M2X or even my Xperia 5 II, both of which provided a balanced listen. The U6t does scale nicely from a resolution and soundstage point of view, width especially expanded noticeably wider on the desktop stack. In terms of noise, using the M2X on volume 0 with the amp circuit active revealed near silence meaning this won’t be a huge concern for this earphone despite its higher efficiency.
Suggested Pair Ups
The U6t is one of the easiest high-end IEMs to drive by virtue of its resistance to output impedance in addition to its sound noise handling. In addition, it is efficient enough not to be too demanding of output power, being serviced well by a smartphone and other low-power portable sources. Of course, you will want a dedicated source of some description to extract the best soundstage and detail performance from this IEM. When considering these pairings, I would not recommend warmer sources as this can harm separation. Similarly, if you are sensitive to treble, the ultra-concise tia tweeter can skew too sharp if you pair it with an already sharper source such as the THX789 or iBasso DX200. Sources that are balanced with a smoother note presentation like the Topping D30 Pro are the ideal match tonally but you do lose a little acuity. I don’t think the tuning is too polarizing so the U6t will play well with most sources, but do keep this in mind if you are especially sensitive to particular qualities.
