circled in red and flourescent green is the input stage gain trimpot. this is accessed by removing the top panel of the rack unit.
this right here may be all the mod you need to up your gain, once you settle on (or even before, but i'd mess with it after and before LOL!) what types of preamp tubes you'll want in it.
i have some other stuff i'll dig up for you if you want.
if you do open your mp1, be careful taking the top and bottom, panels off because there is a lip on both the top and bottom of the front panel that helps keep all the panels flush. Pull the panels out from the back of the unit, don’t lift them out once you remove the small screws that holds both top and bottom panels on.
wtf! pics aren't uploading.
you can see on the top right of the pic, there's a trimpot there also circled in yellow. im not sure what this one does, but i will find out. also, if you do open the mp1, the trimpot in the middle, off to the left of center (but to the right middle of the other two pots) controls the chorus ( i think it controls the overall mix level).
good luck. let me know what else you want. like i said, i have all the mods for this.
http://www.marshalljmpmodshop.net/ada%20mp1%20mod4.htm
you can get the marshall or "mod4 mark 2" mod there.
he also has a ton of other replacement parts/upgrades.
i understand he's really good to deal with. im going to get the upgraded transformer and the kit for it.
G