Well, I gave everything a very close visual check and didn't see any
problems. I cleaned up some suspect solder connections, made sure
there were no shorts or even just some things barely touching each
other and plugged it in. Better, but not there yet. I tapped a few
things here and there and found that the 6SK7 RF amp tube was just not
right. Lots of static when I tapped it with a long piece of plastic.
I replaced it with a tube from my other S-20R and bingo, better than
ever. So I guess visual checks and a bit of detective work solved this
mystery and prevented major surgery. Thanks to all who offered their
suggestions.