Firstly, I hope you didn't pay that shop a cent for their disservice. And I wouldn't let them touch the unit itself with a bargepole either. If they've accepted liability then I'd get them to pay the repair costs of sending the unit to the people who know how to repair it...ie Empeg Ltd.

The garbled display *might* a blown fuse, the fact that the unit reboots itself in the truck, but not on AC suggests that there is a problem with the loom wiring, and given that the shop has just been playing with this....

My *guess* is that the Remote Amp Turn on may be shorted to ground. Are you sure that the unit isn't playing when in AC mode? Can you get a boot log from the serial port?