I have a Bryston 9B-SST five channel amp and a 3B-SST two channel amp. The circuitry is supposed to be identical in both amps. I listen to two channel music a large portion of the time through the 9B multi-channel amp and can tell no difference between that and the stereo sound of the 3B amp.
Bryston amps use individual power supplies for each channel so I am not surprised by this result. However many multi-channel amps share a large power supply and in this case listening to only two channels gives you additional power reserves that may enhance the sound.