Just done comparing the M6U to the M7. To be honest I was taken by complete surprise by the M6U technical performance in comparison to M7. It was so close that at times, I was wondering why they name it M6U – It should be M7 AKM .. The M6U to me is an alternative for musical tuning vs technicalities neutral reference studio quality like the M7.. and yet possesses almost all of those technicalities achievements…almost..hehe..ok let me explain below.
First off, the size of the M6U is way lighter than M7 –on top of that, it’s a slimmer device like the Fiio M11pro but yet a very simple classic design of the Sony WM1a (except that it's even slimmer than Sony DAP). It’s a winner all way around. The weights between them 312G vs 263G may not be much on specs but when I have it in my loose PJ pocket, it matters a lot. The M6U feels so much lighter in the pocket.
I did mention M6U is a more musical tuning among the 2 – by that, I don’t mean it’s a slow-ass amp – It still possesses similar lightning speed as M7 but yet takes off some of the hard edges. The most prominent diff is the mid-range. The vocal stands out more like taking 2 steps forward and cut off some air. This tuning makes the vocal slightly more romantic, and lusher with rounder notes – more forward. Vocal lacks the last bits of air compared to the M7. Which make makes the M7 sound more extended on the top end. (but it's pretty close).
Bass
The M6U mid-bass is a little more pronounced than M7 making it somewhat a warmer DAP. The M7 still has a deeper bass overall and both have similar dynamic power. Both have the same qty but because the M7 is more extended down below sub bass – M7 still wins on the technical level. .. but again it’s a few hairs ahead. By that, I mean if you use a lower /mid-tier headphone like the HD600/650 – it's almost undetectable. Need Arya type to tell them apart. My slow-ass CIEM like the A6T cant detect the deeper bass but it can detect the warmer mid-bass lift of M6U. Overall the bass is slightly slower and decays longer. It works well with some EDM tracks having a hint of boom and not cutting off that quickly. Think of it like adding the fastest tube amp to it. M6U has tubby vocal qualities with a solid state amplifier speed.
Treble –
The M6U takes some of the edges off. It's like what AKM said a velvet kind of sound. Yes and indeed. The tone is more tolerable Ie.. Arya treble peaks. It's not like you add a lush tube amp to it. The diff is not that much. It’s subtle but yet it can be detected even with warmer headphones like Hd650 or A6T. The last bit of extended air is rolled off making notes rounder. The M7 will sound sharper more precise and fast. It resulted in the separation of instruments being more distinct and the space larger between them. I don’t mean the M6U is a blob of sound lumped together, it still has the separation but is just not as precise as M7.
Soundstage.
This is tricky. I truly believe that they are the same width. The problem is M6U may not sound as wide because the vocal is pushed forward hence the background soundstage did not stand out that much. If you play an instrument with no vocal, the width is the same. No diff. but since almost all track has a vocal, the forwardness of the vocal subdue the background music. The M7 does not have a forward mid hence back ground instrument can be heard very clearly making the soundstage larger. . But it's really the same width to me. But what makes the M6U ahead is the height. Since M6U pushes the vocal more forward, the overall soundstage becomes taller (like in a cathedral). It feels very holographic as I immerse in the center of the soundstage, I feel the sound surround & cocoon me very well. M7 is wide but not as holographic and tall as M6U. Overall I love the M6U soundstage. Again, it's not like a day and night diff, it takes a HE1000v2 to really showcase the soundstage. I can still hear it with narrower soundstage headphones but it's not that pronounce.
Overall – I personally rate the M6U as passing the flagship DAP league (albeit slightly) –It's definitely not a mid-tier DAP like the Hiby rs6, M11pro series or IFI gryphon. The M6U technical level is way way ahead of them, it's so ahead that I think it catches the M7 by surprise. The only thing it loses to the M7 is the extension of both bass and treble (again very slight) but then, you get a DAP that is talented in musicality – it made up so well, that I think the M7 and M6U can dance together on the same league all day every hour.
Pic to follow.