Variac or not?

That amp is 45 years old. I wouldn't power it up until it was sent in for a full service and verified that it's performing correctly. The problem with old gear is that some of the parts may not be available anymore, like transistors. pop those or any other unavailable part and it's a paper weight. 

I would power it at about 70VAC for one day.  Electrolyte in electrolytic capacitors eats up dielectric (aluminum oxide) lowering breakdown voltage. Presence of voltage rebuilds this layer.  10 years is about the time I would start worrying.

+1 kijanki! Good advice!

Not.

I used to have 6 of these amps back in the day. They are well built, but on quiet passages, the fans can become annoying.
Before you do anything you may want to pop the hood and do a visual inspection to see if there are any obvious signs of leakage. If you have leaking caps, may want to replace those first.

And be careful using a variac to bring  this amp up.
There is a 10 or 20 watt power resistor in line on the AC mains to limit current inrush when the power switch is first flipped on. If you bring amp up on a varaic, you will burn out that resistor. So before bringing up on a variac, short out that resistor first with a jumper wire. If I had to guess, I would think the main filter caps may still be good, but would be suspect of all the small electrolytics on the pcb boards. Also check to make sure the fan is running when you apply power. If the fan is dead, and does not spin, don't leave the amp on for too long otherwise it may over heat the transistors.