Hi Joeninho,
Sorry I don't have a Marshall at my disposal at the moment, but it all boils down to a matter of taste anyway. My Behringer will emulate a Marshall and I try to keep things fairly "dry" for the most part with just a hint of reverb to get a little more "presence".
I'd give the gains about a 3, and leave the treble around 2, as well as the bass. Granted my guitar is a bit weak in the bass range being fairly shallow bodied. Master volume controls how much you want to rattle the neighbor's windows so use your own discretion on that one, but a 1 or 2 on reverb will give the sense of a larger room with some echo to fill out the sound.
Lots of reverb will get you that "surf" sound and the rest is an experimental thing. There is another post around here that had a fairly good guide to amp settings that I will try to find for you and link you to. Well so much for that notion... failed to bookmark it.
But I think it was in the accessories and effects section? Like two catagories above this one in the index. Sorry I'm not more helpful.... been a long day.
Take Care;
Doug
"what is this quintessence of dust?" - Shakespeare