If you can demo an amp in person, play the amp with the vol full blast, and preamp input paused/unplugged. On someone elses speakers of course....
Noise floor = bad. Leaky caps will be noticable. If not I would't sweat it.
FWIW, I just maxed out my line of credit to buy some used amps in the 12 year old range. Saved tons of cash. Would do it again in a heartbeat.
Usually, you can tell what you are buying and how it was cared for once you see the state of affairs in someones place. Know what I mean?
My dads mitsubishi amp from 1979 still bottoms out drivers if I ask it too, and still sounds great.