What you've suggested Simon is pretty much what I would do if I didn't have access to a variac.

An alternative is to wire a mains voltage 100W incandescent bulb in series with the amplifiers mains, which if there is any short in the amp will cause the lamp to light up brightly, and will only glow dimly and reduce the voltage slightly to the amp if the amp is okay. The best way of doing this is to make a jig with a lamp holder and a suitable power outlet both securely and safely mounted to a small wooden board. Some may choose to include a switch to bypass the lamp to allow full voltage checks without having to re-plug things.

Something to consider... Short of going over the entire amp with all the appropriate meters my usual practice when powering up old devices like this is to leave the amp on for a few minutes during the test... Caps are often a little slow to react to being put back to work.... Applying full voltage for 5 to 15 minutes once all the tubes are back in while maintaining a comfortable distance will inspire confidence that all is okay. Often it may take 5 or 10 minutes for a bad cap to build enough internal pressure inside before horribly and loudly detonating in one of those few moments while you are otherwise distracted..