Although you didn't specify which mark series, Google this:
"mesa mark iii manual".
Scroll down to Maintenance and Service Guide.
This explains which tube does what.
Enjoy.
Also, this:
http://www.thetubestore.com/Resources/Guitar-Amp-Info/Gain-Factor
This explains tube differences, like 12AX7 vs 12AU7 vs 5751, etc.
Preamp or power tubes? And depending on your goals, Mesa tubes are generally a safe replacement for either. But on the preamp side, there are lots of options that can yield some cool tonal differences. Search these boards for lots of in-depth discussions on the topic.
If I can throw another question into this - Can any 6L6 Boogie amp use any Mesa Boogie 6L6 tube? In other words does Mesa bias all 6L6 amps the same? I am using 6L6 as the example because I have 2 6L6 Boogies ( MarkIIb and Electra Dyne) but the question could be about EL84 / EL34/ 6V6 Mesa tubes as well. For the 6L6's at least I know there is a color code. My reason for asking is because I bought a set of TAD 6L6WGC-STR from the Tubestore that are matched for the Electra Dyne but I'd like to try them in the MarkII and the Mesa 6L6's in the Electra Dyne. Thank you.
In my MK1, I use NOS Sylvania 6L6's and Mesa preamp tubes. I also have tried recent issue Mesa 6L6's in this amp and they sound good. For the newer MKV25 I use Mesa tubes across the board.
due to lack of gigging, I have 7+ years of no retubing and now, due to actually tripping over a power cord, I have to hunt down some tubes and maybe a tech