This really sounds like bad earthing on the amps or the empeg itself. It could be that the current demands of the unit going through a bad earth is causing the earth voltage to float up. Have you measured the DC voltage at the empeg 0V point during operation?

You should get the thickest bit of wire you can get hold of (a copper pipe? ) and earth the thing to the chassis with that, making sure you connect to a newly scrubbed patch of raw metal. I really think you must be using the wrong grade of wire here.

To be honest, without a circuit diagram of how you have wired this lot up, this is all guess work. Can you hand sketch a wiring diagram, then scan it and either mail it or attach it for people to look at?

One of the few remaining Mk1 owners... #00015