These originally came with Mesa 7025 preamp tubes in all 4 preamp positions. The 7025 preamp tubes are the same as a 12AX7 tube only these specific ones were tested to be much quieter than the ordinary 12AX7 tube. So some 12AX7 tubes make the grade to be considered a 7025 (or a SPAX7 now).
Now for what tubes to use in your new preamp . . . asking someone what kind of tubes to use is like asking someone what model car is the best or what album is the best ever committed to a recording. There are lots of choices and it really depends alot on what you think is a good sound. Here is a site that explains alot about these tubes and the sounds they can give you. - www.12ax7a.com
Also keep in mind your rig's sound is based upon alot of factors such as
1. Difference in pickups if you use more than one guitar.
2. Cable you use to send signal from guitar to effects to preamp to more effects to poweramp to speaker cabinets (bad cables can create unwanted noise and take out or emphasize certain frequencies which can be good or bad).
3. Equipment/effects in the chain and whether they are true bypass (and whether they are inherently noisy).
4. Your power amp and its power tubes.
5. You and how you play.
6. Settings on treble, mid, bass, tone on any and all of your gear. Even a slight change (especially with Mesa gear) can have a big impact.
7. And other things like strings, scale length, bolt-on neck or set neck, cable length, what you had for breakfast, etc.
You get the idea. Sound is dependent on virtually everything. I remember a story that said Ted Nugent was on tour with an opening act named Van Halen. He tried out the guitarists rig but couldn't get the same sound out of it. Why not? Because he was missing Eddie's hands, ears and soul. Now I think Ted Nugent can rock just as well as anyone but Ted sounds like Ted, Eddie sounds like Eddie and you will sound like you.
I just picked up a Studio Pre and I think it sounds great for what it is. It will probably be my main rig as I like to keep it simple but good. Just like pizza and other good things in life.
Hope I helped a little. I know I probably opened the Pandora's box but information is power.
