OK, I've seen a number of useful replies to my question. I will

• test whether the empeg case is grounded with the RCA cables off


• try powering the empeg direct from the battery


• try grounding the empeg direct to the chassis


• try a 10A power filter I just bought


• try combinations of the above

I have already made a number of cables I will be able to test with as soon as my empeg is returned from being repaired.

But there's one other quetion on my mind: if I power the empeg directly from the battery, will I need to use a relay for the switched power lead? Otherwise, only the permanent power will be clean; the switched power will not be clean. Or does the player only draw current from the permanent power lead?