With 3 amps dying on you, something must be wrong in your system, or house wiring.
What ohm rating do your speakers have? Do they have a phase angle? Did you buy brand new Nad each time or factory refurbs? What specific volume level position do you use? And for how many hours at a time?
It just seems strange to me. I’ve owned 5 Nad integrated amps. 2 C326BEE, one c372, one C375BEE and one C356BEE. And a top of the line cd player, the C565BEE. Three were used. Two were refurbs, and one was brand new. I’ve only owned each for a year or so, but I’ve not had one failure. Now, could I be extremely lucky, and Nad is actually awful, or much more likely, Nad has been around for over 40 years for a good reason, and something else is going on. Still, your experience sucks, and I can understand wanting to stay away from Nad.
PS, the $400 estimate over the phone from the service center, is useless since they don’t really know what the problem with the C356BEE is. It could be more, but most likely a lot less.