Variac or not?
I acquired a Marantz 510M amp that had not been turned on for about 10 years.
Yes a Variac is a must, a little at a time, in 20vac mains increments over an hour to full mains volts, so to "condition (form) all the electrolytic capacitors" again, BUT!!! at the same time with meters hooked up and a scope if you have one you must monitor parameters of the amp while going up in mains voltage EG: dc offset, bias, etc etc and whatever adjustments are in the service manual
Cheers George