We have been using EH 6922 tubes in our T10RB preamps for some time now with excellent results. It is a version of the 6DJ8 tube.
These should work fine in the T8 of T8+ preamps too.
They are shorter than the 6CG7 tube so will fit with the cover on.
Your preamp will run cooler than with 6CG7 or 6N1P tubes.
We have them available for $35 a pair plus $10 shipping.