Speaking of batteries, the battery in my AM503B is weak (.6V, should be ~1.5V), so I removed it to see what it is (390 or SR1130SW). Removing it cleared the NVRAM and reset the amp to factory default (355 error after power on).
You had mentioned before that the AM503 normally starts up with ref coupling, but mine had always defaulted to DC, never ref on start. After reinserting the dying battery, now it starts up as you said in ref coupling. I wonder what other weird things were set. 🤷 Anyway, I ordered a new battery.
Thanks,
Josh