I can definitely believe the sound is exceptional from the M15HD2/M25 combo, NAD does engineer great analog sound quality in their products. I really love the M25 amp, I owned one a few years back. However their HDMI-based processor products have exhibited inconsistent performance (going back to the T175- to current product-line). Although I'd have to see one (an M15HD2 that is) in-person before I make any judgements.
I owned one of the original-release T175 processors, it exhibited many of the well-documented issues of the day- ie. popping transients (switching sources, or enabling STOP->PLAY->PAUSE->etc.,...). Most of this was fixed with firmware combined with some circuit fixes(resistor/cap networks). This was all related to improper muting circuitry. After 2 or 3 firmware upgrades my unit functioned mostly good for a few years. Analog sound was exceptionally nice. But I wasn't happy that my other down-stream gear (amp, speakers, etc.,...) suffered these random transients/loud pop's. I've since moved on to Marantz AV7005 and AV8801.
Anyway, I am surprised to hear/read that owners of current NAD processor products are still suffering major issues. I would work close with Bob Moran (if he's still there) to make sure every issue is properly addressed.
As big a company as NAD is I don't think they have the kind of money to throw at developments like their more commercial counter-parts (Denon, Yamaha, Onkyo, etc.,..), it takes quite a development team to get all the digital processing refined.