I'd have a close look at the wiring of those paralleled 10 Watt resistors up at the front of the chassis on the offending side. The high voltage coming out of the regulator is unusual, but perhaps there's just no load on it.
I would also expect exactly the conditions you described if the jumper from NEG OUT to -reg, or -reg to the center of the 9 pin socket, or the black wire that leaves that socket up to the front of the chassis has a flaky joint. That would remove the ground reference from the board itself, which would allow for some strange voltage readings and no glow from your 300B on that side.
-PB