Hey RG, frustrating when you can't get a tech interested in helping
. My tech has given up on my MP-1s, but they are much more complicated than a power amp, particularly as they combine digital switching etc. with the analogue audio circuit.
The main things that come to mind for an older power amp is:
Tubes (obviously) and the higher plate voltage may wear them out a bit faster. And as you say new tubes will need to be biased properly.
Electrolytic capacitors from that era, the liquid inside them hardens over time (whether they are used or not, it just hardens) and they get noisy, hence the foundation of the noise mods in the preamps, the mods are predominantly changing out old electrolytic caps. For a power amp, the power supply caps often need changing to fix hum and noise issues.
Pots and connectors, generally a good clean, de-ox etc. or replace if they are badly worn from use