Just for clarity Upscale Audio sells most of its tubes, even the rarest, to anybody. The reason for the hold on certain 6922 and 7308 types is most dealers are out, and we were going to run out in six months. As it is now we sell them only if you buy gear here and will have stock for 7-10 years.
Poke around Dave for Amperex 7308. Or maybe Siemens 7308. I wish I could have helped you, but if you persevere you can find some.
That preamp is a clean slate. Very neutral. Low output impedence. If your system is exhibiting a certain balance you don't like, treat the system in the most cost effective way.
That would be a pair of tubes and speaker placement.