Are you 100% sure that the empeg ground is connected? If you're grounding through the audio grounds, and this is the only ground, the tracks aren't really big enough to take it and it will cause audio issues.

You should, as with home use, not be able to hear any drive noises on the audio. I can plug the empeg in at home, pause it, turn the amp (80w rms) to max and not hear drive noises with my near next to the speaker cone.

Can you check the alternator?

Hugo