Platypus said:
V1 is the most important as far as tone.
This is not a true statement for everyone. I'm not trying to put you down, but just trying to expand awareness here. Everything matters.
It has been my experience that a well-"tuned" amp will be sensitive to
any tube change in the signal path. And amps with rectifier tubes can respond differently when changing to other manufacturer's versions of that same type rectifier.
Try different tubes in all positions, and use your ears to decide. I've found that changes in phase-inverter tubes, tone circuit/driver tube, etc., have a bold impact on the way the amp responds, as well as the V1/input tube. How you use your amp makes a difference, as well (do you get any power section clipping? how clean do you play?).
If you haven't experimented with tube changes other than V1, you may be in for a pleasant surprise when you do.
Cheers!
- Thom
p.s. Doug Preston, at Doug's Tubes, is a great and wise resource!!