Hey, crazy thing. In my last reply I mentioned how I wish something like the AMT SS-20 were MIDI switchable. Well it turns out it is. You need to buy an add-on called the AMT FS-2MIDI.

The preamp does not have every sound imaginable (just 3 tube channels) but I've found that a good tube preamp, my volume and tone knobs on my guitar can create amazing variety. Right now I'm using a 2 channel combo amp with eventide boxes in the FX loop, so this is a step up. 

The SS-20 is a pretty high-gain, "rocker" preamp, so if thats not your style, an Ibanez tube king has a very close circuit to the old Chandler Tube Driver used by Eric Johnson, but it's not MIDI capable.