Ask yourself what you makes you think the tone is bad in the first place and work back from there. Is it too bright, too fizzy, too bassy, too honky, not enough gain? An EQ pedal would provide more benefit than screwing with a distortion pedal your digitial amp may not play nice with.
Also, what are you comparing your tone to or what artist are you trying to emulate? Don't compare the tone your getting from the amp to what you hear from your favorite albums. A miked amp tone with all the recording studio eq'ing/wizadry is quite unobtainable in your bedroom (unless using cab/mic simulation software).
A good practice with digital amps I've found is to go through each amp model with just a touch of reverb with all tone settings set to 12:00. From there weed down to 2-3 amps (Clean, crunch, high gain) that you like best. Then tweak tone/gain controls for those as necessary to sweeten them further. Try to live with those 2-3 amps for atleast a month and play the guitar vs. tweaking knobs to no benefit to your playing. You may find that the core tonal eq structure of the Marshall isn't for you, but give it some time.
What kind of guitar are you using? Strat with single coils vs single cut with humbuckers makes a huge difference.
Goodluck and take notes as prior poster mentioned.