The JMP-1 has a handful of good sounds. I've found that the main sound is very unique and can't be changed too much. I've used it with an ADA tube poweramp and a couple of 2-12 Celestion loaded cabinets. I use US Strats and ESP strats with hums mainly. You can get a good Metallica type sound out of it, or a more well rounded GNR type tone out of it. I personally like a lot of midrange in my tone for a funkier tone and for leads that cut through the mix better. I'm not in to that scooped mid sound. I recently sold it to buy a Triaxis 50/50 setup. I miss the JMP-1 for a few reasons -- it was easier to program and it had a better direct out and speaker simulator signal for recording. However, the Triaxis sounds warmer and thicker. The JMP-1 does one great distortion sound, the Triaxis does about three great distortion sounds, in my opinion. The JMP-1 has clean and fast channel switching, the Triaxis has some latency in switching (I think due to all-tube circuitry versus the tube-hybrid circuit of the JMP-1.) The significant difference I've noticed between the two is that the JMP-1 is bit more sterile than the Triaxis. By sterile, I mean that the JMP has a refined crispness and consistency to it that is great for metal types. The Triaxis, however, has more rawness to its tone -- it has an uncontrolled feel to it that gives it more character and lends itself to more interesting guitar styles and tones. That's just my general take on the two.
If you need MIDI channel switching and want a dependable rack unit for under $700, this is a great preamp. If you don't need MIDI, but want Marshall tone, get a Marshall head or combo. For the money, the JMP-1 is better than anything in its class, tube or not. If you can afford the Triaxis, I'd recommend it instead -- you'll get more distinctive sounds and wider tone pallette.