How would I check the Ground Plug functioning properly?

The hum is only coming from the speakers - not the unit. With only the amp turned on and only the speakers connected it is fairly low hum (almost ear pressed to the speaker low) but once the-prepro is plugged in and devices are added via HDMI connections it get's louder and louder. It does not seem tied to the cable input from DirecTV satellite, as disconnecting/reconnecting the cable makes no difference.

If it is the amp I will flog myself for not pursuing more deeply and settling for the 'balanced cables' fix.