One thing to remember: V3 and V5 have cathode follower circuits which will cause premature failure on most preamp tubes. So far, the Mesa (or JJ) 12ax7 and a few of the Chinese tubes (Penta-Labs, Ruby, Preferred Series) will operate quite well in these two positions. Tubes manufactured and marketed by New Sensor (Tung Sol, Sovtek, Mullard, EH, and many others) will not do so well and will fail in V3 and V5. Explore with your hearts content with any tube in V1, V2, V4, V6. I found that the overall tone of the amp can be modified with just a change in V1. V2 will also influence the character but not as much as the primary or first stage tube. However, I have experimented with a JAN/GE 5751 in one or the other positions and found the end result is interesting. I did not notice much of a change between the Mesa (JJ) or Chinese tube in V3 and or V5. Note that the tone controls are post gain (V3 tone stack).
As for power tubes, have fun. I am currently using the Ruby version of the Mesa 6L6GC tube, as it seems to have a different gain characteristic with a bit more presence than the Mesa sorted tube even though they are essentially the same tube (or are they?). I have tried Tung Sol 7581 (not bad but the tubes were used up in my Mark V), TAD6L6GC, and SED 6L6GC. I actually liked the TAD6L6GC in the Roadster, but yet the saturated gain character of the Ruby or Mesa tube is good too. I actually like the darker tone of the Ruby or Mesa tube than the brighter SED 6L6GC. To each their own when it comes to character and tone that you desire. I have also tried EL34's and their variety (GL KT77, TS EL34B, Mullard EL34, SED EL34, Mesa EL34, and a few more I forgot what they are.) Since I have a Mark V and RA100, I have many tubes to try out (assuming they are still any good, the Mark V seems to eat everything up quickly except for the SED 6L6GC).
For classic rock, the Vintage or RAW modes might be where you want to run the amp. Try experimenting with the tube Rectifiers too which seems to reduce the bass a bit. I am not all out for Metal but generally stay in the Modern voice settings on CH3 and CH4. There is also CH1 Tweed or CH2 brit, with the gain maxed out on those two channels, you can get some really great crunchy tones there.
