Solved! Dave at davidreaton.com suggested that: (1) the outer silver ring might be bound and could be carefully pried out; (2) the ball bearings of the vernier mechanism might need lubrication; and/or (3) the tuner shaft might be bent and in need of straightening. The silver ring was indeed...

I'm hoping that someone with knowledge of the inner workings of the Advent 300's geared tuning dial can help me out. I recently purchased one of these amps sight unseen from someone who just had it completely restored as follows: -- all transistors, caps, bias diode replaced -- no-hiss mod...