Thanks for offer, Murrel. I found the culprit. It's the projector. When I plug in the projector, which has a 3 prong plug, the hum starts. It's certainly a ground loop. I've heard bad things about cheater plugs, but is it all that dangerous if I try a cheater plug from the projector into the power conditioner that the projector is plugged into?
I have run cheater plugs many times. I understand the potential risks. We use them all the time in pro audio. In fact I have probably 200 plus at my brew and blues pub. I use them to eliminate hums on stage. No one has ever been electrocuted.
Here is a suggestion that has worked for some in the past.
If your sub is close to the receiver it is a easy thing to try.
Make a single wire with crimp lugs on both ends (forked style). hook one end to the ground (phono or antenna) on your receiver and the other end to one of the amp mounting screws on the amp plate. make sure that you bare the metal under the mounting screw. I personally have never had a hum so I have never tried this. I have had a few customers say that this has fixed the low level hum that they had.
It is your choice with regards to the cheater plug.
Let me know if I can help