I'm by no means a tone expert, but mess with the equalization a bit. You don't want the mids scooped (low mids, high bass and treble), but its probably gonna be somewhere close to that. Mess with it until you get a distinct "crunch" instead of a muddy distortion with no bite to it. You also may want to mess with the gain or drive a bit to get the distortion really tight.

Somebody else could give you better advice I'm sure, but thats where I would start.