It appears that you are closer to confirming that it's 60Hz hum from the power lines. If you turn up the volume on the sub, can you hear the hum?

Also, try turning off all the lights to see if the problem disappears. Also unplug all of your equipment from the walls. If it goes away, plug them in one-by-one to identify the offender.

You and Sirquack should get together. You can fight over who plays Sherlock and who plays Watson .