Ok. Last word on the subject for now (ty for entertaining me):

I found that the ground for the tone knob is on an eyelet that connects directly to the chassis. I have also run a ground wire from this eyelet to the preamp ground section (which includes grounds for the input, 1st tube and volume knob).

I probably created a ground loop this way. I will remove the grounds from this eyelet so the tone knob is only grounded alongside the rest of the preamp section and report back later when I have time... 🤞

I was suspecting the tone knobs ground already because with no input signal, the buzz strengthens when the tone knob is adjusted.

Oddly, the buzz disappears when the tone is all the way up and strengthens when reducing the tone downward.

I'm excited to have a lead to investigate. Ty again for your support.