Just took a look at this today, and that's DEFINITELY a possibility. It's what I was thinking was a problem right before I got to your actual question on it.

I had something similar in an old Mercury Tracer (no relation to me - haha). I had installed a head deck and amp, and long story short, it turns out that the piece of metal in the dash that I had grounded the head deck to was in fact isolated from any other metal parts in the car. The head deck grounded itself through my RCA shields. I could tell once I realized that when I disconnected them and plugged them back in I could see a tiny spark!

Properly grounding the head deck took care of it. I suspect your empeg is done the same way. Obviously it has a wire available leading right to the battery, but - I would spend some time making sure the empeg is actually using that ground path and not something else.