I am not sure if this could work , but here it goes.
take the firstamp model in the amp_model_dual preset and convert it into a preamp the output from that would be sent into the second amp model and then out from there. That way you could use IR's generated from the preamp sections of you favorite amps. If you IR a specific preamp at its various levels you will get a better distortion.
Here is a paper on modeling a Fender Champ amp.
http://cegt201.bradley.edu/projects/proj2003/dtubeamp
and here is reverb coded out.
http://www.music.miami.edu/programs/...wne/thesis.pdf