I have an AR rather than an MD, and I’m not particularly familiar with dubstep or dnb, but FWIW when I want a fat snare I usually start with a Linn LM-1 or an Oberheim DMX snare sample and then roughen the sound up, and also add lower tones from the analog engine.

EDIT: OK, I just listened to some dubstep and the snare sounds like it might have a TR909 sample as its base sample.