I think there are two pickups you might want to look into (besides the emg81, wich rulez!).
Dimebag Darrel uses the Bill Lawrence XL500 (bridge position). I've heared these give a similar sounds as the emg81's, only a bit less clean.
You were asking about Dimarzio's, the X2N has most output of all dimarzio's.(and has a clear sound)
These three pickups are similar in sound and amount of output. (not the same, but similar!)
If you're also going to change the neck pickup, make sure the difference in output of the two pickups isnt too big.(any "high power hb" wil do i think)