Anyways... finally got some time tonight to do some work on the motherboard.
First,
populated the PSU section to see if it works... yup, A.O.K. (had
to jumper the switch temporarily since the motherboard isn't hooked to
the switchboard.)
So far so good, the switchboard, 48V section, and now the DC-DC section are all working fine.
Work done so far... all resistors, zeners and diodes stuffed.

Tomorrow after church service, I'll solder the caps, IC sockets, pots and switches and XLRs, then start testing.