The last time I auditioned any amplifiers was at the local McIntosh dealership in 2006. Listening to those amps of that era I did notice subtle differences in various models of their solid state amps.
I'm told now the current incarnation of the MC275 actually has bass now. I lot of people have this amp from various decades and love their tube sound.
Since I haven't heard the mc601 all I can say is the mc611 takes 15-20 minutes to warm up, and they drive the blades with impressive tone and dynamics.