Problem solved. For some reason, the remote line was not getting to the amp. It must have been mis-wired/shorted somewhere. We cheated, and powered the amp remote from the ignition, so it's always on when the engine is on. I didn't feel like ripping into the dash again to fix this. I'll do that when the tuner module is ready.

This should have been an easy problem to debug. But the thing that misled us was that the Empeg display did not go on in this situation. So I assumed the problem was with the Empeg unit, not the amp. If the Empeg display had worked, then I would have immediately deduced the problem on the amp side.

BTW, the power to the amp is wired correctly: directly from the battery.