What were the best years for McIntosh power amps...

Lots of McIntosh bashing here. I’ve had Pass labs, ARC, NAD, Counterpoint, Yamaha, Marantz, Pioneer, and others. Some of these were very good (Pass) and others not so good. ARC sounded good but was unreliable as hell. 
In terms of sound quality, reliability, and holding it’s value McIntosh bested them all. IMHO.

This audio stuff is all very subjective and system dependent.
Listen, and buy what you like would be my advice.