64Audio Duo Review – Fun, Forgiving, Fabulous

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. I gave the Duo 100hrs of burn-in prior to review to ensure maximum performance during my subjective analysis.

Tonality –

The Duo is a bass-biased U-shaped monitor at its most fundamental. Colouration doesn’t stand out as offensive in any manifestation with progressive emphasis and no major peaks or troughs. As the midrange is clear and slightly vocal-biased, they don’t sound overshadowed but treble and bass especially do tend to draw greater attention. Bass emphasis has been executed in a slightly different manner to their BA earphones too. The Duo has less sub-bass focus, being warmer, fuller and heartier; the dynamic driver providing greater slam and pressure than their BA models too despite some measurable roll-off at the very bottom. To my ears, this is desirable as it prevents bass from overwhelming and delivers a good mix of texture and impact. Due to the midrange tuning, the earphone sounds clear and impressively clean overall but well-structured and coherent with increased upper-midrange smoothness and density. Its treble is airy and articulate, showcasing fairly typical tia frequency response and note presentation characteristics beloved by many with a slight bias towards the middle-treble.

Bass –

The Duo is a reasonably bass forward monitor though the emphasis is executed in a manner that avoids obnoxious colouration of the remaining sound. They have a deep/mid-bass emphasis with a slight sub-bass roll off mostly likely a result of their open design. As emphasis tapers off through the upper-bass into a slightly recessed lower-midrange, they aren’t exceedingly warm or stuffy. While lows are full and warm leaning, as peak emphasis lies just below the mid-bass, the note structure comes across as emboldened rather than exceedingly round or tubby. Note thickness is increased and the low-end is generally well-weighted. Unlike the similarly open-back Shozy Black Hole, the Duo has power and impact in spades – and the open-design need not be a worry for bass lovers. It delivers a well-defined rumble and plenty of rich dynamic driver texture in the mid-bass. The Duo also delivers a decent amount of pressure, but sub-bass doesn’t announce itself with the same physicality of a sealed DD IEM.

In turn, outright note weight and slam doesn’t strike as a defining aspect of the Duo’s low-end delivery. These qualities play well into the general note delivery of this IEM which strikes a considerate balance between impact and body. Perhaps as the driver must bear the burden of both bass and mids, the Duo has a slightly longer decay than your average high-end DD IEM. At the same time, it does have an impressively assertive attack and strong control. In tandem with the fuller tuning, the Duo has reduced bass separation but retains more than ample note definition due to the quality of its driver. Its lengthier decay and style of tuning imbues textured and fleshed out notes in a manner that feels intentionally whimsical. It doesn’t strike as the paciest and cleanest bass delivery but is unabashedly powerful and textured with no pretence of accuracy. In turn, though separation takes a hit, bass is just responsive enough to keep up with complex passages whilst imbuing sizeable impact and engagement.

Mids –

Admirably, the midrange retains a highly natural voicing in all regards. Emphasis climbs gradually until a 3kHz prominence before falling off into the 4kHz region. Their slightly laid-back lower midrange aids separation, resulting in a medium warmth. As colouration in both the bass and lower midrange has been executed in sensible moderation, the net result is a natural presentation that remains so on a variety of musical genres and mastering styles. By extension, I didn’t find the Duo to be remotely over-warm or congested, albeit, surely not as revealing as many high-end monitors around this price. While separation still inspires and pinna gain is appropriate, the Duo is defined more by its smooth and coherent character. At the same time, its vocals do have some precedence over midrange instruments, avoiding bass and treble dominance.

Their increased density is mostly responsible for this smoother quality. It mitigates intensity and strain whilst promoting a more coherent and forgiving listen. The effects of this tuning also aren’t felt too much in terms of overall openness due to the 5kHz treble peak. With a smooth upper-midrange to counterbalance, the lower-treble never exacerbated sibilance to my ears. They have a hint of glossiness due to the treble tuning which brings small details a shade forward and retain admirable clarity despite the focus on coherence. This means that though midrange resolution isn’t quite on the level of similarly priced models such as the U6t yet alone the company’s higher models, definition remains quite good throughout. The tuning is altogether in good taste if you don’t mind the laid-back presentation. The Duo is a more forgiving sound from 64 Audio that retains a highly natural voicing and structured yet articulate nature.

Highs –

 Meanwhile, the top-end is defined less by its incisive leading edge and crisp lower-treble but an airier middle-treble with greater shimmer and air. While its airiness isn’t overt throughout due to its bass leaning signature, the top-end itself does benefit from a good amount of headroom and energy with a slight lift in overtones. The lower treble remains nicely detailed and focused with a small 5kHz bump, albeit, you get less focus and “tia magic” than on the similarly priced U6t which appears crisper in this regard. However, I do feel the tuning makes sense here in the context of the rest of its sound, the focus on the middle-treble helps to open up the presentation and avoid bass dominance. The actual treble voicing is relatively thin and shimmery, in turn. Notes aren’t especially well-bodied nor are instruments flattered with huge texture and richness. Instead, the Duo is delicate and open, showcasing a very clean transient response but a decidedly more atmospheric style of presentation with less emphasis on bite and crunch.

It has strong top-end extension as you would expect from the company, and even a good amount of sparkle at the very top. Foreground detail retrieval is just above average in class, hampered by the thinness of its note delivery, but background and micro details are brought more to the fore with aplomb. It also does so without being exceedingly aggressive; treble emphasis is executed in moderation which keeps the top-end mostly in-line with its generally smooth and forgiving character. In turn, though the voicing is energetic and sparkly, the positioning isn’t exceedingly forward, avoiding excess sharpness and fatigue. While I do think 64 Audio’s other models are a better showcase of their tia tweeter, the Duo does benefit here from its pristinely clean note delivery and abundant headroom that plays sensible complement to its open design.

Soundstage –

Incidentally, the Duo doesn’t strike me as having the largest soundstage and appears to have a depth focus to some degree. Of course, this is relative to other high-end IEMs that, around this price, can be quite formidable at times. The Duo is an average performer in class as far as width is concerned but commands impressive depth, especially with its grand low-end presentation. Imaging is quite sharp too; directional cues aren’t highlighted but are still presented clearly due to the small lower-treble bump. Layering performs well, they aren’t especially well-delineated, but the presentation remains well organised and composed, with no smear evident even on complex passages. The weakest aspect of the Duo’s performance is with regards to separation. They are not a poor performer here, but their coherent bass and midrange voicing means notes aren’t surrounded by much air either. Its large notes fill its moderately sized stage leaving just enough room to discern small details, but they aren’t clearly isolated as on IEMs with a more revealing tuning. Given the level of bass emphasis, I would consider the Duo to perform well, but I wouldn’t go into this expecting  

Driveability –

The Duo has a relatively low sensitivity for a high-end IEM but can still be considered an efficient IEM overall with a 98dB sensitivity and low 9-ohm impedance. The upside to this is its linear impedance design which means the low impedance doesn’t cause many issues with regards to source pairings.

Output Impedance Sensitivity

True to their marketing, the Duo isn’t especially sensitive to output impedance and sounds similar from a tonality point of view from the vast majority of my sources. I confirmed by switching between the Shanling M2X (1-ohm) and Hiby R6 (10-ohms) which revealed that the Duo had minimal frequency response variation between the two when taking source colouration into account. The Hiby did sound slightly thicker in the bass but was, otherwise, very similarly voiced. The same rang true for my Xperia 5 II smartphone meaning listeners can enjoy a consistent sound profile from a wide range of sources, simplifying pairings.

Driving Power

Ultra-low impedance IEMs do usually enjoy a bit more current output meaning a dedicated source is a good idea. While LID does mean you the overall sound character is consistent, the Duo does scale nicely with a bit more power. Specifically, the bass performance was markedly better when listening from my THX789 as opposed to the M2X, both having identical output impedance and volume matched to the dB. The M2X was balanced with a well-resolved treble, but bass was less controlled and articulate, with lower definition in general. The THX789 sounded quite a bit more separated and also pushed the stage much deeper. As the sensitivity is not too low, the Duo isn’t especially prone to hiss either. There was no audible noise on the M2X with the amp circuit on, but volume set to 0 as is evident on more sensitive IEMs.

Suggested Pair Ups

The Duo thanks to its combination of LID and lower sensitivity manages to sidestep the challenges of driveability on a multi-driver platform, especially a low impedance one. In so doing, it remains fairly efficient but isn’t sensitive to hiss and has a consistent FR from almost all sources. In turn, source noise and output impedance should not be seen as limiting factors here. The Duo does scale nicely with good amplification that said. Specifically, a good solid-state amp will provide a more articulate and controlled bass which is essential to separation.

For portable users, a more powerful dongle like the Hidizs S9 would be appropriate, and listeners can then enjoy a scaled up experience on their desktop setup. In terms of colouration, the Duo definitely benefits from a cleaner source given its already bassier character. While this will always be a preference, I did find warm leaning sources to introduce too much fuzz and fullness into its image.

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