The empeg audio outputs are not opto-isolated, no. I don't know of any head units that do have opto-isolated outputs. Take your average sony/kenwood/etc and you will find the audio outs are grounded to the chassis.

Generally, the aux *inputs* of a head unit have floating ground with high CMRR input stages - this subtracts the ground reference from the signal and gives you clean audio, no matter if there is noise between the receiving unit's power ground and the sending unit's audio ground (which sounds like your problem).

Have you tried any audio ground loop isolators (not ideal,obviously) between the empeg and the becker?

There was a boxster with an empeg install (+eq+amps, no becker) which cambscar did - it had awful noise on it. They bought it round here and I couldn't find out why. In the end it turned out to be the alternator had a bad connection - it was completely silent after this was fixed!

We do have a pretty low noise floor - lots of people have commented on it. It does look like the becker's aux in stage may not be ideal, though. The becker/changer setup is a designed-in system and most likely *does* have high CMRR inputs.

Hugo