Short update, the replacement transistor arrived today at last. Installed and checked, weird output channel DC voltage is gone, so I'm starting the reassembly. Hopefully all goes well, so many wires and screws to put back in now. Fingers crossed.
EDIT: Bleh. Still a no go. I got one "I PRT" (current protection) and since then, "DC 0" again. So maybe something went overcurrent, blew that transistor. I'll pull it all apart and start checking things again in the next few days.
EDIT2: A little more testing today, showing -6vdc on all the outputs on MAIN(1) now. More checks later, been busy lately.