[Techman]

I've just this moment remembered something about these amplifier head units. Correct me if I'm wrong, but doesn't the earthing in the pre-amp head unit rely on all the front panel pots. I seem to remember that there's earth bonding to the tin plated covers of the pots that in turn relies on the crimped round tabs to the actual alloy casting of these pots, and thence the nut and washer on the spindle connection to the chassis. I once had a problem a few years ago where there was either noise or hum (or both) caused by either relaxed connection or oxidation between the crimp tab of the pot casing and the actual pot body. Going round them all with a sharp screwdriver blade to dig in and confirm a connection between tab and body will confirm (with the unit running of course). Crimp them up with long nose pliers or replace them if this doesn't work - remember what you've learned about earth returns, so you can't just run another bypass earth. It may not be what I've just said, but worth checking out just in case.