Overall comment is that this unit does not have too much bottom end. Starting with the clean channel 1 (the cleanest), one can never achieve the bass depth that a Fender Twin or a Boogie Mark I can offer. Similarly, the lead channels (OD 1 and OD 2) are not too bassy either; I actually found it easier Boogie sounds (i.e., midrange heavy) than mid-scooped sounds in the overdrive modes.
The difference between clean channels 1 and 2 is mild. Channel 2 is quite a bit brighter than one, but they break up in much the same way. So, I do not see too much use in having two different sounds is they're going to be so close to one another.
As far as the overdrive sounds go, they are good. OD 2 is the more aggressive of the two and it gives quite a bit of additional distortion and more trebble attack. One complaint in this department, at high gain setting this preamp hisses very loudly; of course, once a note is hot the hiss disappears in the mix, but it is A LOT noisier than a Boogie Triaxis at comparable levels.
Overall, this is a "one-sound-preamp", changing the settings will change the coloration of the sound, but not the nature of it. Other preamps do experience a change in character with different settings, but the JMP-1 is not one of them. If you dig the Marshall sound, you will like the JMP-1, but if you're looking for an array of sounds, think it over before getting one.
Reliability was 100% for me, however, the Q.A. at Marshall may not be so great. My first unit had a deffective rotary knob (which changes the patch numbers, or function parameters), such that it would skip numbers when turning it. I went back to the dealer, got a new JMP-1, and the knob worked great. However, one the push-buttons used to select trebble, bass, etc., was kind of sticky and would "almost get stuck" when pushed in. Another trip to the dealer and another JMP-1; eventually, my personal disatisfaction with the sound (as expressed above) caused me to sell the thing all together.
I would not buy it again because the sound didn't suit *my* taste for the money. Compared to other similarly priced units, the Marshall seems to be a good choice. But, and I repeat myself, the Triaxis would be worth the extra money in my opinion; however, I have come to believe that racks are a nuisance to carry around and that in the end, the best sounds I have ever gotten have come from amplifiers and not racks.