Hum in speakers, and suggestions for stopping it

Hi, everyone.

Any success stories in scenarios similar to mine, described below? Grounding boxes? Outlet upgrades? Gadgets like the Morely Hum Exterminator (née Ebtech Hum X)? I’m looking for a non-invasive solution (e.g., not getting a dedicated line into the room).

The situation: I had each of my PS Audio M700 amplifiers plugged in to a Stellar PowerPlant 3. The system sounded great, but I knew I'd likely get better sound with the M700s in the wall outlet. I made the switch and there was a huge improvement -- so I want to keep this configuration.

However, there's now a hum in the speakers. Given that there was no hum with the M700s in the PowerPlant 3, I think it's safe to assume the problem is the wall outlet. I can't hear the hum 99.5 percent of the time when music's playing, but ... well, you know.

"My" electrician came out yesterday and said doing a dedicated line to the outlet would entail demolition and/or other options that aren't feasible right now.

Thanks much!