[Al Green]

Figured it out. Turns out that when I replaced C206 I connected it to the lug for pin 6 of the 6J5GT, rather than pin 5. Lug 6 is unused for the tube, but acts as a tie point for where the radio signal goes to the bandswitch and C201 (part of the negative feedback?). I moved the C206 to pin 5 and correct volume operation was restored, and it was significantly louder than it had been before, which is no wonder.

Thanks for the help with this.