It would appear as if the Heil 400 lives.
Happily sitting at idle.
I grabbed a used toothbrush from my tool bin and used its somewhat rubbery handle to nudge the offset trimpot into as near a null condition as the trimmer would allow.
0.4 millivolts, I can live with this.
Sadly, I don't have enough dummy load (yet) to really fire this thing up and watch it work, nor have I speaker loaded or o-scoped this amp yet, but based on DC voltage measurements alone it appears to be functional. There's a repair tag hanging off the power cable stating "A channel out", which happens to be the channel that has been rebuilt in its entirety. I think it possible that in our communal music store hang-over we took that to mean that an unspecified channel was broken, and into the pile it went.
I'm having a difficult time locating much information about the amp on the net. It does not appear to be a Heil branded Phase Linear 400, as the Phase Linear PCBs look different. Nor is it an Ohmega 400, as the faceplate is different and evidently the Ohmega uses odd-ball output devices. This amp has MJ15003 outputs, which appear to be in production even today. So, if you're reading this and have hard data on this amplifier, please let me know.