The breaker for the outlet that the sub is plugged into may be on the other phase/rail in your main circuit breaker box, or the outlet could be wired out of code. Either of those could cause the hum also. If floating the ground doesn't work, check that all the room's outlets have a black wire to the brass screw and white wire to the nickle screw. If so- and you find that the breaker for sub's outlet IS on the opposite rail from the rest of the system; move it to same rail as the rest of the room's outlets. If floating the ground DOES work: PS Audio offers some nice cords with removable ground prongs, ie:(http://www.musicdirect.com/product/74108) an inexpensive one. If you haven't been using an upgraded power cord, you'll reap some sonic benefits in the replacement process. A cheater plug will take you in the other direction.
HELP: how to eliminate speaker HUM?
I've developed a nasty, fairly loud hum from all five speakers in my HT setup as well as my sub. If I turn off amp, all humming stops. If I disconnect IC from preamp, that channel/spkr hum stops. Is this ground loop hum? (I've heard of this but don't know what it means). I've heard there may be devices that stop the hum ("humbusters"?). Please help. Thanks..Jeff