Just bought - Polk Audio RM6750 Black 5.1 CH Home Theater Speaker System, and registered it today. Am now waiting for support to answer me back...Monday. Anyway, the sub hums when plugged into an electrical outlet, by itself...6 different circuits (3 were GFI), hums even when it is turned off (if hooked to an outlet)...hums when connected to Yamaha amp. Only way I have been able to stop it, is to unplug from electric outlet. I like the other speakers, but didn't buy a surround sound to listen to a humming woofer when movie 'Bass' sounds are slow or idle. My Vizio soundbar had a 6" woofer that works fine. Apparently hookups are not the problem, since it hums by itself the moment its plugged into electricity. New at this (other than the Vizio soundbar) so have no clue why a speaker would hum.