There was a poor solder joint at the bottom left-most (closest to the 9-pin socket) point of the headphone jack. There are 2 black wires connected to it and a repeat of the "pencil test" with a bit more vigour revealed consistent crackling when either black wire was touched.
I have plugged in proper headphones finally.
I'll use it for a bit, then swap some tubes, then put in the Speedball in a few weeks.
Thank you again for your help. It has been humbling and rewarding.
Best,
Chris