It could be an issue with the PCB. Ive found that many amps with a PCB have hum issues, and its often times because the heaters get delivered to the tube sockets through pcb traces, rather than twisted pair wires. it could be something as simple as the heaters inducing small voltage somewhere in the early preamp stage, which then gets amplified. I wish I still had my Vt-22 so I could test that hypothesis.
on the other hand, there is a reason people tend to use DC filaments for early gain stages.
on the other hand, there is a reason people tend to use DC filaments for early gain stages.