Buzzing Rel subwoofer

I realize this issue has been raised in previous threads, but these threads are years old now and reading them didn't seem to yield any solutions. I bought this Rel S-2 new about 2 1/2 years ago and lately it has started to emit a 60-cycle hum. It's not a loud one and it's affected by volume changes but it shouldn't be there. The sub has spent most of its life plugged into an APC power conditioner, which itself is plugged into my system's dedicated circuit. I've tried plugging the sub into a different wall socket on that same circuit but it behaves the same--still hums. Disconnecting the speakon cable has no effect either. Any ideas about this?
My REL subs are a Q150e and a Q108e MKII…the Q150 had a little 60 hz hum that was eliminated by utilizing the ground lift switch above its AC cable socket, and the 108 (bought later) was always humless so no need (that one doesn't have a lift switch anyway…I guess you have to pay more for that). Ya never know...