When I'm in the lab at work, I can't bring guitars, so i simulate a guitar pickup at the input with a 6K resistor in series with a 1mH inductor in series across the input terminals. Signal can then be injected in the junction between the two. This yielded results similar to my previous post.
The results of my work so far resulted in the humming being almost gone, and the squealing frequency moved up to around 12KHz.
Yesterday i replaced the BJT and JFET in the input stage, and that cured the squealing somewhat. With nothing across the front input terminals, there is still squealing, but as soon as there is a pickup (or my pickup simulation contraption) connected, the squeal disappears.
There is still some humming at high gain settings on the dist channel, but i think this might be a mechanical issue, as the transformer is humming quite a bit, and i can literally feel the vibrations from it in the chassis. It might be time for a replacement transformer?