There are mods for the MP-1 out there too. If you're looking to buy, make sure you're getting what you want, either modded or stock. The Recto mod, the 3TM mod, noise mod, 3.666, a bunch more. Here's a link with some info.
http://adadepot.com/index.php?topic=148.0
Voodoo Amps does a nice MP-1 mod. Been a long time since I've heard it, but I remember it being pretty badass. Trace does good work. Looks like Voodoo doesn't do the mod anymore, but you can find them out there on the used market.
The biggest issue with the MP-1 is finding replacement parts. They tend to break down, and the parts can be hard to come by. Not impossible, but a bit annoying. If I recall correctly, the jacks on it tend to crap out pretty easily. I'm no MP-1 expert...trying to recall info I picked up 10 years ago.
I remember a guy from the Metboards way back in the day who got great tones out of a 3.666 modded MP-1. Turns it into more of a Recto feel than the stock Marshall tones.
You can get a lot done with an MP-1 and a parametric EQ in the loop. Empress makes a nice and simple ParaEQ pedal. The EQ can really help sculpt the sound and get more versatility out of it. It's a very cool rig. ADA made a rackmount graphic EQ themselves, the MQ-1. It's MIDI controllable, which is really cool. A graphic would do the job too. The MP-2 had a graphic EQ on board, if I recall correctly.
Most of the time you hear people running MP-1s into Marshall style power sections for the obvious reason. I wonder what it would sound like with other power amp tubes. 6L6s, KT88s and the like. That could make for an interesting sound!
The clean sounds are pretty great too. Spanky, chimey 80s cleans. It can be set to be squeaky almost-direct guitar clean (think early Dream Theater), or you can get a bit of grit out of it. Should work well as a pedal platform in theory.
Another suggestion, try some different tube types in it. 12AU7s perhaps.