Sounds to me like you're experiencing a ground hum. I recommend getting a Klein outlet tester from Home Depot. They're cheap ($10-$12 depending if you get it with or without the GFT checker for outlets with a breaker button).
Check your outlets for good grounds and proper wiring. Try plugging one of the speakers into a kitchen or bathroom outlet and see if you still have the hum. Even old buildings that have been converted from 2-prong to 3-prong outlets should have grounded outlets in the kitchen and bathroom (where there's running water).
I also recommend a Furman power strip to attenuate EMI/RFI. $36-44 for a 6-outlet strip from Furman with a 15 foot cord and worth every penny.
