AudioGal
100+ Head-Fier
The 6CG7 is pin compatible with the 6N1P, amplification is a little lower, plate (anode) resistance is higher. Heater requirement and pin-out is the same. So it could work. I have some, when I get them out of storage and am able to try them, I'll report back. Interestingly the 6CG7 is just a 6SN7 in a smaller envelope with a 9 pin Noval base.I have read a suggestion for the EH 6CG7 in the Valhalla. I never actually tried it when I had that amp, but some people seemed to like it.
The 6N6P doesn't really have an equal. Or one that comes really close. The new ECC99 seems to be the same tube with a different label & number, but I'll need to buy a couple to confirm.
Guess it's time to build a tube curve tracer.