One thing does occur to me now that I think about it. Does this effect occur when the empeg is completely shut down, ie with the ignition off?

It only happens when the empeg is either playing or sitting in standby. If it's completly off, or pulled, the problem goes away.

I'm going to dig into it a bit more. Xanatos here on the board has a RioCar installed into a 2000 LS series Saturn, and dosen't see the problem, so something different between the LS and SL that I have might be what is causing it. I'm going to see him later this week to look into a remote problem, so I will probably try a swap of the units to see what happens.