Hey Jeff, welcome to the depot, I see Dante got you registered

, excellent.
The MP-1 was the first midi programmable tube stereo rack mount preamp ADA made. 3 voicings Dist tube, Clean tube and SS (solid state cleans), active eq (bass, mid, treb, pres) mono series fx loop, stereo chorus...
They also made lots of other things we never got nor really heard about in Australia (combos, pedals, rack fx etc.)
Later they made the MP-2, with lots of extra goodies, 10 tube voices (no SS), wah, tremolo, stereo parallel fx loop, stereo chorus, noise gate, cab sim recording outs, midi continuous control (CC), macros..
Seems many found the MP-2 too complicated (I loved it and now use MP-2) so they made the MP-1 classic.
The MP-1 classic goes back to being more simple like the MP-1 voice wise (albeit different sounding to the original) but kept some of the MP-2 additions e.g. stereo parallel fx loop, cab sim outs, maybe CC control?. I'm not sure how long after that ADA had a fire and went bust but the Classics never came to Australia, and they are harder to find (and a prick to work on is my understanding).
The 3TM mod is an extra tube for the original MP-1, 1 more triode gain stage and a cathode follower. Popular with those who wanted more gain (IMHO it had heaps of gain already but some wanted more

, (then the MP-2 has sooo much gain, lots more than the stock MP-1..and needs the built in noise gate to tame it)). So the metal guys like the 3TM, but you loose the Clean tube voicing, not so clean anymore.
The majority of mods developed were for the MP-1 for various reasons, check out Schematics & Modifications
http://adadepot.com/index.php?board=22.0 .
Hope you find a good one for a decent price

. IIRC you are in Europe? so need a 230v model. ADA preamps are all country voltage specific i.e. US 120v, Japan 110v, Europe/Australia/UK 230v.