I own a 4B3 and it's the best amp I've had. I preferred it to the Pass X250.8, not just for lower heat, but because I found the Pass a little too lush and tubey. A great amp, to be sure, but not as neutral to my ears as the 4B3.
Regarding the Magtech, when I auditioned it a home, I disliked it intensely. It was the anti-Pass -- icy and uninvolving. I can't explain why I reacted that way to this widely respected amp.
As I said in a previous post, I have enjoyed both Classe and Bryston amps (the brands you were considering).
In getting what suits YOU, it would be great if you could audition what I think is a prime contender, the 4B3 (or 14B3). Any way you could audition one? They are great amps, more relaxed than previous Brystons. You can find dozens of glowing reviews in actual publications, as well as by happy owners.