I always find that if there's any kind of lowpass or high end rolloff, you'll have to turn that off becuase of the EQ curve they use on Ownhammer cabinets. I always end up shutting off one of the EQs, or messing around with the highs on the graphic EQ. Rosen Digital IRs are a little easier to work with existing presets, I've found.

Best thing to do in my opinion is to record something and reamp it until it matches the original preset more or whatever recording you're going after, or better yet record some tracks and get your own mix going, then reamp while making changes. So that way, if you like it but know it's a bit "dark" and no longer mix ready, you can still have fun playing with it as is, and worry about EQing (even changing gain with different cabs). Some cabinets actually turn the rig into a higher gain (or lower gain) one. The newer Ownhammer cabinets are even labeled this way now and that helps a lot.