Depending on the amps age it may be a good idea to check the internal wiring.

Bend the wires, especially close to the valve bases and make sure the insulation doesn't crack.

Caps can be a pain, especially electrolytics. Larger caps usually require reforming regularly during storage but probably not applicable in this case.

Alternatively, take valves out, fit RCD breaker, turn on and cross fingers--3 outcomes : bang, burn or nothing