The most likely reason for hum with an old power amp is tired (old) electrolytic capacitors in the power supply. Replace them. If there still is a problem then look into other things.
Transformer Hum
I own a few vintage amps, two Citation 16A's, an Ampzilla, a Son of Ampzilla, and a recently aquired AEA 620. Is it normal to have transformer hum coming back through the speakers? Could I have other issues like RFI from a LCD rear projection tv or interconnects that aren't sheilded well? Or is the age of the equipment the main concern? Then there is the grounding issue. I don't know where to start. Thank you for any thoughts or advice.