Glad to be of service!
Not to long ago I had an amp that had a hard plastic knob with rusted set screw. I tried everything to remove the screw, heat, cold spray, liquid wrench, elbow grease, etc. I eventually stripped the slotted head.
So I ended up putting a large C-clamp on the knob, squashed the knob and broke it to bits. I'm glad the pot shaft was metal or I could have broken that as well. Luckily I had an exact match of the knob in my parts drawer.