The FAQ entry was where I found the Crutchfield document. Your installdr links are stale and I meant to mail you about that.. Take out the /InstallDoctor/

For a ground loop to exist, something has to be multiply or poorly grounded, yes? Well, the first post should have covered that, but...

The amp is installed under the driver's seat and the ground is about a foot long; screwed into the chassis and yes, the paint is scraped. Attempting to replace this ground by grounding direct to the battery changed nothing.

The ground from the Empeg is 6 gauge and it's not grounded anywhere other than where the amp is. Nothing else is touching the chassis and removing it from the dash and letting it hang changes nothing.

For kicks, removing the tuner module also changes nothing, so it's not antenna noise:-)

That exhausts everything from the InstallDr links except EMI noise. EMI noise shouldn't change depending on how the Empeg is powered.

I don't see that there's much more I can do to remove the possibility of ground loops; Certainly nothing in those documents suggests anything other than ground loop isolators on the RCAs, which I suppose I could try, but the likelihood of the RCAs having a lower potential to ground than the 6 gauge wire grounding the Empeg to the same ground the amp has now seems a bit far-fetched.