Armed with another variac I resumed an easy ramp up of voltage to insure everything was copasetic and wouldn't you know it, about 50 volts in I hear the "pung" of inrush current draw. Explicative deleted. So, it's NOT the test gear (except, of course, for the one with the faulty outlet). Troubleshooting ensues.
Amp was tubed with dummy load and the stand-by switch was set to play.
1st step: Remove tubes & repeat procedure... "pung"
Explicative deleted. After checking over my work and running continuity tests across the circuit to ground (both of which passed) my thoughts drift to the power transformer.
2nd step: Fire it up in stand-by to see if it's a shorted primary. Acts fine. What changed?
Hmmmm. Continuity check on switch reveals that while the pilot light side switches as intended, the center tap to ground portion (switch is pictured in the stand-by position, nearer half under scrutiny) exhibited high resistance when it should have been a dead short. Several actuations confirm the switch is garbage and needs to be replaced.
So, I theorize that the "pung" at the variac was instigated by arcing across the switch feeding inrush to the capacitor bank of the B+, and it probably wouldn't have been noticed if I cut the amp loose on full mains voltage instead of pussyfooting around. This also provides a satisfactory reason why all my voltages were wonky at the sub 50 volt position of the variac.
I did ponder the option of simply cleaning the switch contacts, since I have access to them.. but somehow every time I think along those lines I am hounded by the image of villagers with pitchforks and torches. Best to swap the part out entirely.