By my standards these amps have enough OD on tap... but if you need more, I would suggest on using some booster pedal.
Are you sure you tried everything... turn up the volume pot... and OD trim with PAB on and all that... still not enough
+1 , I agree with Mcinku these amps have tons of gain, too much sometime's given the right setting.. when I want my OD to cut through I use a Analog Man Mod Boss SD1 ( Tubescreamer variant ). Use a boost pedal and you won't have any problems.
with the OD trimmer you should be able to get gobs of gain.