I finally (3+ days) figured out. I pass along this update in the hopes that it helps someone else out. The source of the hum wasn't grounding at all. It was airborne RF given off by a Kodak camera dock in a distant room. All I can figure is that the electrical circuit for that device went through a wall behind my stereo system.
A big thank you to all those that responded.
A big thank you to all those that responded.