Nick :
Possible problems/solutions:
When you 'programmed' the DMD/MF setup, were your trims set dead-center neutral? They need to be set dead-center during the programming phase. Also make sure that any ratchet-center sticks start and return to center during this phase.
When you run the beast, did it not run true (ie with a centered steering stick the tank still turns at speed) You do NOT correct with the trims, as this will interfere with the programming. You need to adjust the 'neutral' pot on the DMD. Its the balance of power for the left & right motors.
Check for anything that may cause the gearboxes to bind. If you pulled the motors for upgrading or reduction installation, make sure they spin freely and with equal ease. You may have one pinion set closer to a spur, and this would cause the motor to need more force/power to run, and cause the turn. Also someone noticed that some reductions a pinion projects too much, and a spur grinds on the pinion.
Lastly, check your track tension... is it equal on both sides? This, like the gearbox movement, would cause the tank to pull to one side.