Have you tried unplugging everything in the shared circuit to see if there is a specific culprit? Put a meter mic on the speaker and walk around unplugging stuff with a stopwatch. If the hum exists in the wall but not through the power plant, my would be there is noise on the neutral or ground and it’s finding the biggest transformer tap in the whole house circuit to land on. The benefit of a dedicated circuit - since all circuit neutrals come together in the panel anyway, is really the ground between the panel neutral/ground and earth / waterline. You need to get that noise to have a preference to go elsewhere. Cheater plugs - and I believe hum eliminators - are basically lifting the ground. If you can find the problem, you could put a cheater plug there and not at your amp.
To me, a cheater plug is a simple test. I’ve been lucky in my house that I haven’t had this issue - but I have chased it through hospital construction sites before. I actually had a project where we solved the problem by installing a large, dummy load transformer in an IT closet. The noise on the line preferred that load to the IT switches. If you have a spare amp lying around that has a larger transformer than your amps - try plugging that into the same circuit.