Answered this the other day when you asked a very similar question.
Use the overdrive channel of your amp and put an overdrive pedal in front of it, but DON'T turn the gain on the pedal up. You just want to use it to hit your pre-amp with a stronger signal to drive the tubes into distortion sooner.
Leave the gain on the pedal almost off (1 or 2 at most), turn up the volume until there is no discernible difference in volume between pedal off and pedal on, hit a chord and set the tone knob of the pedal where it sounds good to you, and let the amp do the rest.
You really shouldn't need a pedal, but if you choose to use one the best way to do it is to use the pedal as more of a boost of your pickups' output.
Have you ever used your Marshall in the context of a full band? Like, a drummer, bass player and maybe another guitarist? If you haven't done so yet, you may find that when you do your tone is TOO distorted and gets buried in the mix of everything else.
Also, dial some midrange into your tone. It's more important than you think it is. You don't have to crank the mids, but don't scoop all of it out either. I like mine set at about 4 or 5.
I suspect part of your problem lies in 1) Not setting the EQ up properly, and 2) Your playing style. A lot of guitarists don't realize it, but how you hit the strings is a HUGE part of how you sound. Proper picking technique can make an amp you don't think sounds aggressive into a monster.