The 20 year warranty is irrelevant!!!
Yes, it is always mentioned because it is unusual, but who would want to listen to poor sound for 20 years? Not I.
On its merits, the 4B-ST is a powerful SS amp that is very neutral yet smooth, and takes control of the bass with athority. It matches easily with other equipment, and in my experience doesn't have such a profound sonic signature that it masks other parts of the "food chain". In my system (Rega Jupiter, ARC SP9 II, Oracle Delphi MkIII, Magnaplanar 2.7, and my room!) it sounds great. I found the McCormack's rolled off and lacking punch (in my room!). Compared to Classe' the Bryston is more athoritative and better at unraveling detail (in my room!).
It is a wonderful sounding amp: big and authoritative, detailed yet warm (not euphonic, but warm), and revealing without a hint of hardness. To my ears it is not brittle, strident, or etched in any way. FYI: I listen to just about everything from classical to grunge, but mostly spin 50's & 60's Jazz.
The warranty, however, is horrible, it flutters in the breeze, crackles when folded, and is wrought with surface noise. Don't listen to it!
Brystons are worthy products on their sonic merits and like every component; subject to the associated components, source material, room, and sonic preferences of the listener.