+1. Almost certainly transformer. Do you hear the hum when you're close to the unit, through the speakers, or both? Any which way, transformer. Could be DC on the line, could be ground loop, could be a transformer that hums. (Other manufacturers when asked have replied: yes, they do that.)
Tube Question
I have an amp that has Russian Tubes. This amp purchased in the 90's has always had a hum to it which when playing music isn't heard. I'm thinking of buying 1 6SL7 and 1 6550EH tube to swap out and see if I can find if one of the tubes is the problem. The 6SL7 is Sovtek and the 6550EH is electro-harmonix. Should I replace with a similar tube from another company or continue with the same company? Is this approach to finding the problem sensible or a waste of time?