Amps Recapping

Take a look.  The usual recommendation, and it's a good one, is to use a variac to bring it online the first time. 

Sorry I should have added more detail.  Take the cover off  and look for signs of corrosion/leaking around the electrolytic caps, or signs of bulging. 

Caps made in 2000 or later have a much better reliability in terms of aging on the shelf than those before.

Assuming it passes your visual inspection then use a variac to raise the voltage from 0 to 120V slowly.  Turn the power off at the first sign of trouble.

Also, connect sacrificial speakers for the variac test.  Any weird sounds and stop your testing.

I will second @erik_squires recommendation about using a variac.

A couple extra thoughts:

They are probably OK after 20 years since it has been boxed, but prolly will not last long.

Do expect it to sound weird when the voltage is low, there will be a voltage threshold things have to cross before they start to function.

You can get a very cheap variac on Amazon for under $60.00.
It is very important to de-rate random Chinese stuff like this by 60% to 80%. These items are often fused incorrectly and will short the hot leg to the frame when the fuse blows or when they melt.

So a 5A variac is really only good for 1A to 2A. They are basically a huge variable resistor, so they less voltage to let through them, the hotter they get. If you let full voltage through, they will not get hot at all.

A 5A version safe enough to test a 5A fused component provided you do not let it idle for too long at partial power. Make sure you unplug it before touching the metal frame if the fuse blows. I have the 20A version and have let it idle for quite a while on a 4A circuit with it only getting very warm, which is exactly what you would expect by de-rating it 80%.