The M6 is sitting on my desk now. Not bad all things considered: it's as compact I'd hoped, works with W10 with no issues and although there was a mini-DVD with software included in the box, all the relevant WHQL stuff got picked up by default, so there was no need to touch it except for a few Foobar2000 plugins. Got both the EU and the UK type plug with it and the required USB cable, but no 3.5mm to 6.5mm jack adaptor, which comes with almost all decent headphones these days but may be an issue for some if one is expected in the retail package.
The chassis is plain, with a neat little screen on the front displaying all the essential info, and has a solid feel to it and rubber feet to prevent sliding. The joystick control we covered already: some will like it; some will hate it.
With a friend who knows a bit more about electrical wizardry than I do, I took a gander at the internals to check the workmanship and spec -- no issues or messes there; a pretty tidy unit, really.
Going in blind, I find it hard to distinguish the sound from similarly priced internal sound cards, my old ODAC + O2 combo and the more expensive Fostex beastie that has given up the ghost given my DT770s and cheaper gaming headsets. With lossless content, I'm actually enjoying it quite a bit more, but this could merely be a novelty placebo.
Anything ~300 Ohm or less should be absolutely fine with it, as per spec, and I'm not really planning to run my next HP upgrade off it anyway or large speakers, but for science of course I'll give it a whirl to see if it's capable enough to stretch its muscle a tad further without severe degradation in sound presentation. Of course, given the inputs and outputs, it can also be paired with other kit/amps should more power be required.
Overall, I feel no need to fiddle with it much and it's just sort of there, smoothly doing what I need it to do for now, offering good sound, no gimmicks and portability with a few convenient connectivity options + an integrated HP AMP.
Let's hope it lasts a while.
It'll be replaced eventually and overthinking it for the money isn't worth the effort. If it's cack, I'll fish around the used market for something better.