I've probed the current monitor with my scope and the it looks like EMI to me. I've debugged further and noticed a peculiarity.
There's a reed relay (Fraser, can I post the schematic?) K1 that ultimately switches the L2-Current an L3-Focus outputs. It does so via switching the wiper output from the L2/L3 pot to an opamp.
The voltage before the relay is nominal. The voltage after the relay is much higher -- I've measured:
Wiper: 1.31V, node after relay (R5/R3) 3.3V
then 1.47V -> 5.26V, then 1.56V -> 6.08V
The inverting output is at most 2.3V, so the opamp goes to its max possible output of 10.5V pretty quick, which corresponds to about 9.2V on the BJT emitter that is upconverted to the tube control voltage.
I need to take out the gun board and probe some more, but it's a huge pain to reach the bottom srcrews...