This is where i was not happy with this preamp. It all started out great but this it one of those amps that you hear about that gets worse and worse as you use it over time. I tried everything to get it sounding good agian, replacing tubes, cleaning contact points, resoldering joints, ect. But no luck. also this amp had its days; some days it would sound ok, then other it would sound like shit. When your doing as much gigging as i am, you can't have an amp thats tone always changes. Also, as a whole this amp is built for a one guitarist rock band. On the best sounds that it can get, it fills up a very wide range of frequency range. If i wanted a good tone, it ment almost pushing my rythm guitarist out of the mix because he just couldn't be heard. And its not the i was louder than him by any means, its the fact that the "bass shift" really does what it says and wayyyy too much. It gets your tone very bassy and very punchy, which is cool if you by yourself but not if your playing with other people that like hearing them selves. Also this amp cuts like none other. U can completly cut the mids and it will still cut through the mix like a knife. It would be perfect for leads but dont even think of playing rythms on it. Also, as a general rule with rack gear, if you want to sound good, do not spare any money with cables or any components. A shitty cable or component in the path with the best preamp in the world will make your tone go to shit. And with your jmp 1 already sounding like shit, u better get some better gear to put in the path....if you havn't got this thing yet, i wouldn't, theres better things out there.
Read the sound quality description. The tone changes where unbearable. But on the plus side, this thing never actually completly failed me so thats something to be said i guess.
for the price, you can get something off of ebay that could rape the hell out of this things sound. I think marshall needs to update their rack gear a litte because there are lots of improvements that can be made on this and many of their other rack units. And for all you kids that think this stuff is good cuz the pros use it, check out all the other stuff they use along with it. And dont get the idea that this is anyones primary amp. This amp has maybe 3 acceptable tones that could make any kindof professional recording. Other than that, you better hope you have moeny left over to get something else that will fill the tonal void this thing will give you.