I've got some very audible hum, looks like this at the speaker inputs:
which seems to be due to the audio from my set top box being brought over to my pre-amp, with the loop being completed by a recent new piece of gear (also plugged into a line input on the preamp) with a 3-prong plug.
Anyone ever use one of these Hum-X things:
and how well did it work out?
They aren't exactly cheap, but if it works...