Definitely a ground loop problem, I think it commonly happens when hooking up to TVs because they get plugged into another outlet or surge suppressor (often they were not installed at the same time.)
Saw this problem when connecting my Tivo (audio only) to my Outlaw 1050. Very confusing, since I even got the hum when selecting other sources.
My other thought is that the ground loop comes not from the power supply in all cases but sometimes is associated with the incoming cable signal. I have seen that in other (non-Outlaw) setups.