Yeah, Kempers from my knowledge just eq match and then model distortion based on modifying the basic sigmoid function of the distortion by taking impulse responses (the funny wee woo noises). I actually did a thesis on guitar amp modelling and its pretty simple really.
Axe FX also models distortion in the same way but runs on a cascading gain structure much the same as how when you add boosts or overdrives into an amp it reacts to the boosted signal.
This more accurately represents the affect that each section of the amp has on the sound and allows for seperate modelling of the pre and power amp sections.