If these are for your 115 Mark II combo, I have a couple of thoughts.
In my Mark III and Mark IV amps, I definitely prefer the Winged C/S.E.D. 6L6s, and their EL-34s in the Simul-Class sockets. They are getting expensive and hard to find, but really like them for their durability and tone.
An ideal situation would be to find some of the old Sylvania STRs that Mesa had them build to their specs. NOS, probably impossible to find, and they would be VERY expensive.
Mesa amps are non-adjustable, fixed-biased amps. However, many of those older amps were modified with bias pots. This a VERY suspect "improvement" in my mind. If it has not been modified, and the bias circuit is still in spec, you should be able to use Mesa tubes, and they'll drop right in...no re-biasing necessary. They're good tubes, and they have the best warranty in the biz.
Unless you're having some specific preamp tube problems, such as excessive noise or microphonics, I'd leave them alone. Preamp tubes last a long time. The Mesa and E-H tubes work well. Using all JJs sound a little dark for my taste, but have them in some sockets, and they work well. And I will add that I would not use the JJ power tubes in these amp. They're great in Marshall heads, but they don't hold up very well in the high powered Mesa combos.
I've had good experiences dealing with thetubestore.com and Doug's Tubes. And Mesa, of course. I can recommend that you call them.
Bill
