I replaced some small value bipolar caps, and the one near the horizontal driver transformer. Same issue. (didn't replace any resistors as I haven't found more bad ones YET)
https://youtu.be/P8dgQ33JAhQ
Although it's hard to see in the video, the picture shrinks (and sides curve inwards) slightly as it warms up. So I think there's still an HV/power supply issue.
Oh, and I recapped the "channel selector" board & audio circuit (couldn't replace a few caps though). Now there is no sound, but I can still hear the amp working (so in other words it acts as if I don't have audio plugged in).