After a long, long sleep: ML-335

Have it looked at first.  It's an expensive amp to repair so a little caution is warranted.  It doesn't have to be an ML service center.

Don't plug it in and turn it on. You will need to invest in a Variac. Set the Variac to 5 volts (AC) and turn on the amp and leave it at that voltage for at least six hours and then increase the voltage in 10 volt increments, with each increment at least two hours. This process will reform the power supply filter capacitors, which after 25 years are almost certain to be "dried out".

Or you can recap the amp. Pyramid Audio was quoting $1400 for cap replacement more than five years ago. 

+1 for the Variac

I would probably just plug it in and let er ride! But…..

I think GS5556’s suggestion is the better one and you should do that. 

The big issue with old amps is the caps going dry, leaking, etc. and that’s what the variac should help you with.  That is, they'll help you blow a fuse before you do too much damage.  When those caps go bad they can take out the rest of the amp with them. 

Before using though, open it up and take a look around the biggest caps you find. They’ll look like soda cans with screws. Check them for any sign of corrosion or leakage. If it looks like the Coke or Pepsi has seeped out even a little, or the cans are bulging have it serviced first.

Do the same for any of the smaller caps too.  Inspect them for obvious bulges or leaks and at any signs of degradation take them to be serviced first.