Can confirm that this pot workaround and recapping the audio board fixed my buzz as well. Since I had the monitor out, I recapped the main pcb too. Recapping made it so I can actually center the projection on the tube now too.
Traded for a Super Punch-Out!! over the weekend and the buzzing was bad from both of the amps on the 20-Z2AWs. It took more time removing the PCBs than it did applying the fix (new 50K pots).
It worked great on both, and the hum is practically nonexistent. Thanks again @sand1.