The Adcom 535 I got around the same time runs my home office system, which I use when doing stuff like this. Neither is as accurate as the Audio Research amps I use on the top end or such older amps as the D-150--wish I still had one of those!--but they run and run and run and never give me a problem.
So yes, some older amps are just fine. This is not to say I would not buy a new amp given the $$ and desire, but since this is not a priority for me right now--fixing up my newly-purchased house is #1--I am not in the market.
But those two amps seem to do the job for over 40 years, so why complain? What else do you use that is 40+ years old and works as well as it did when new? Maybe a musical instrument, and of course my C-2, which needs constant attention, but that's about all I can think of.
Cheers!