From what I've read, this issue is specific to certain types of 12AX7s made by New Sensor (branded Sovtek LPS, Electro-Harmonix, and Tung Sol RI; possibly Svetlana and Mullard RI as well) that don't follow the 12AX7 spec properly WRT cathode-to-heater voltage and can have problems as the cathode follower in Marshall-type designs where there's ~160VDC difference between the cathode and heater. (As I understand it, the 12AX7 spec says up to 180VDC between cathode and heater is allowed, but apparently these tubes only tolerate about 100VDC.)
This is definitely a question for PRS customer service.