I just put a complete set of JJ tubes in my Mesa Mark IV that I got from Bob at Eurotubes http://www.eurotubes.com
I play mostly blues and classic rock
and these tubes make my Mark IV just nail these types of tones. I got a matched set of EL34s and 6L6s because I have always like that combination. I got some really nice Stevie Rayesque tones with my Strat in the neck position - I use EMGs and they can sometimes sound glassy on the clean channel - but these JJ's seemed to have warmed that up a lot! I am pleasantly surprised. Can't wait to go to the blues jam this week and open her up!
Most importantly both my gain channels are much quieter now! (It was pretty bad I had not changed any of the tubes in about 2 years...)