Power amp humming - help please

The cheater-plug will help troubleshoot the hum. If the hum stops when it’s plugged in then there’s a ground-loop.

To review, you disconnected all audio cables from the amp and you plugged the amp into different receptacles.
It’s possible they may be on the same circuit. Last thing to try, plug the amp into the refrigerator or air conditioner receptacle. They are possibly on dedicated lines, different circuits than your audio system.

Do you live in a house with your own service panel (circuit box)?

Test the amp with a cheater plug, an adapter which removes the ground.
https://www.target.com/p/general-electric-2pk-grounding-adapter-polarized/-/A-14442316?ref=tgt_adv_X...

If it removes the hum then you have a ground fault somewhere.