dual sub set up, one hums but not the other?

[blue11]

I couldn't find one of those cheater plugs at the Walmart or Home Depot around me.
I cleaned up the wires a bit but the hum is still there. I disconnected the cable tv but the hum did not disappear. I played a dvd that had some bass and checked both subs to see if they both hummed. Only the DSP hummed.
I'm going to switch the sub cables again and see if that solves it. if this doesn't work then I'll try and get a ground loop isolator.

any other thoughts?

EDIT: switched cables, same result.