As near as I can tell -- and someone correct me if I got this wrong -- but the new Bassman 500 hybrid is, in essence, a Bassman Pro front end (tube) mated to an ICEpower module. With the old-school look, of course.
My other take is that the audio engineers tried their best with the bigger Rumbles to model the front end of those old tubies we love so much. And the ICEpower is so flat it just pumps out a larger version of whatever you feed it. To the degree that the engineers were successful in modeling the tube pre, that is what gives the bigger Rumbles such a successful tone.