I'm probably one of the rare few here with the M1.
Current combo is Audio-gd DAC19 10th Anni and M1HPA (older one, not HPAP), and I gotta say it's really an amazing combo.
From my listening and the limited online reviews, I think the online reviews are pretty close, so can take that as a general ok reference... things like (off my memory) neutral-with-a-touch-of-warmth, plenty of power, slam/authority, musical, non-strident treble etc.
I had the M1DAC previously, again the "neutral-with-a-touch-of-warmth" applies and this combo has been with me for around 3-4yrs. It was replaced by the DAC19 after I decided to try R2R 6 months ago, and has never looked back since. They stack together quite nicely too.
The M1HPA benefited readily with just entry level aftermarket power cables (over kettle cords).
I also had the X-CAN V8P and WA6SE. The V8P is a slightly different flavor from the M1HPA. Can even be a bit variable with tube rolling. I retained the M1 mostly because the V8P did not have a power button
... The M1 is overall cleaner and faster.
Perhaps a better idea of where the M1 stands is comparing it with the more well known WA6SE. To me, the 6SE (stock tubes) is slightly more powerful, has more impact/attack in bass/treble. It is almost 80% more expensive than the M1 but I think the sonic difference is just a touch above marginal. Then again, not everyone will see this difference as "better". It's subjective after all. I still prefer the overall package of the M1... the form factor, "maintenance-free", cleaner/quieter compared to tubes, more laidback sound.
As for the Black Cube Linear, I have briefly heard it but it was at the store and was noisy, so as good as nil. Anyway from online impressions, it was deemed to be more towards the cold/strident/bright side. So it all depends on what headphones you are using.
The combo drives mainly my HD595/598/598SE/600/650/800, K702.65, Focal Spirit Classic, Spirit One S, Momentum 2, CD900ST, and I would also plug in my PX100-II and Porta Pro. On IEMS there will usually be some hiss. To give a sense of power, driving the 300Ohm Sennheisers to moderate listening volumes requires the volume knob to be at around 9 to 10 o'clock position. The notorious AKG requires just about the same.
In any case, the best is if you can demo somewhere as your ears are the best judge.
