An update on this project: The amp is turning into a serious puzzle. The tube section is totally weird. a quote from M. Mardis:
" Hi Shep,
> > So you gave up on the idea of replacing the opamp with tube?
No, that's what I'd like to do. But the way the tube is used in the amp it has no votage gain. In fact it attenuates some. It is used to supply more drive current, then the opamp steps signal voltage up and passes it on to the amp chip. It would be nice to get rid of the op amp and use just a tube, but that may mean another tube and/or reworking the power supply. Just time and money - that's all. I have some tubes and circuits to try, will let you know. I hate to have to rebuild the tube power supply, but might have to."
If it comes to it, the tubes will have to go. I'm getting impatient!