Hugo,

Yea, if I disconnect that ground (coming off the loom) the Empeg won't power on at all, so it's gotta be grounded. I have it bolted to the chassis.

Interestingly, my ohmmeter says that there is a ~7ohm path from the Becker aux-in to the chassis. From the empeg outputs to the chassis is 1 ohm. I get continuity tones from both.