Ok. If I unplug the sub's coaxial, it's dead silent. Once plugged I get the hum.

I unplugged the sub from the wall outlet and plugged it in to my Monster Cable (I know...) power center and it has gone down significantly but it's still there. Unplugging the coax going to the STB helped a bit more. So I guess I would need one of those Ground Isolators, but that doesn't seem to fix everything...

There is a ground logo next to a regular screw behind the EP350. I'll try removing it...