First of all, DON'T get a 2:100, that was designed with the Rectifier Recording Preamp in mind. The 2:90 is the one configured especially for the Triaxis. Also, go to Eurotubes.com and email Bob. Ask him to quote you for a full retube of both units. The tubes Mesa install are cheap as hell and far too cold. Bob will hook you up with the same set used by Petrucci, Hettfield, Satriani or whatever sound you specify. As far as a midi foot controller goes, the standard for the Triaxis is the Abacus.
Prices (I've spent the past few months hunting the same rack down) are as follows:
Triaxis V.2 with Fat Mod $950-$1100 depending on how old it is and whether it has been retubed or not.
Simil-Class 2:90 $650 - $850 depending again on tubes and age.
I did a lot of research and most independent reviews say to go with the 2:90 rather than a Marshall or someother poweramp. I ran my Triaxis through a Samson Servo 170 and it sounded crap compared to the 2:90. No comparison.
You should also look at getting a noise reduction unit as the Triaxis is quite noisy 9AC hum and electrical frequencies). The BEST you can get is the ISP Decimator. It's the latest from the guy who invented the Rocktron Super Hush C but is leagues ahead in technology.
Good luck on E-Bay!!