Transformer Hum

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 is usually heard at the transformer. Hum that you hear thru speakers is frequently caused by grounding issues, RFI, and quite often something as simple as cable (mis)routing. I had it happen to me once by passing an IC next to a line conditioner and on another occasion looping a bit of IC under the amp (actually under the power transformer of the amp).

IME the easiest way to solve the problem takes a bit of time but should work the first time out. Disconnect everything from the amp (except the speakers) turn on the amp. Do you hear anything? No, then add one item back into the system at a time until the hum reappears. When that occurs you should be able to source the problem, its either in the component itself or the cable route/grounding of that component.

Good luck......Oh, I forgot to mention that some times the hum can be sourced to the house current and can be removed by an appropriate filter.

And Eldartford could be right, but since that costs money I would defer repairs until you have eliminated the other possibilities.

As Newbee sez...a ground loop or interconnect alongside a power cord could cause hum, but I assumed that you have ruled such things out already. Evidently your other vintage amps don't exhibit the problem.

The Citations exhibit the most hum and the AEA the least. I will start by process of elimination but I suspect I have a combination of the problems you both have mentioned.