AAWingnut,

I had a ground loop that caused humming through my self-powered subwoofer and it hummed whether my HT system was on or not. It turns out it was my cable TV line. I purchased a Jensen Transformer VRD-1FF Cable TV Ground Isolater that installs on the cable before the cable connects to the cable box. After I installed it the hum was gone. If you have cable try disconnecting it to see if the hum stops. If it does the Jensen device might work for you too. Other ground loops are more difficult to diagnose; I got lucky.

As to why the hum just started up; (if it is the cable) maybe the cable company did some work in your area recently or maybe you have a loose cable connection somewhere.

I purchased the isolater from an E-bay store called Cable Solutions for $50. An alternative might be a Panamax or Belkin surge/filter. They might have ground isolaters in them too.

Hope you find the solution quickly.