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I would break the connection between the plate of the 57 and the 1 meg pot and insert a capacitor there, to take the pot out of the DC path. Likely a .05 would be ok in that location.
Are you positive that the speaker voice coil is not rubbing slightly on the magnet?
I would also try a larger value for C-9 for a more mellow tone... perhaps experiment with different values for the most pleasing sound. A "voice/music" switch, with 2 different caps might be fun to incorporate.
What station is that on 620? Many talk/sports stations are over-processed for AM broadcast these days to overcome interference... they don't sound very good, but they can be heard better in fringe/noisy locations.
jr
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