Chances are good that the hum is a ground loop. You can test this by using a "cheater plug" that bypasses the ground pin on the plug.

Chances are pretty good that it's being caused by your cable input to the system, further evidenced by the "Pure Audio" mode on your Onkyo bypassing the video circuitry.

::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::