It's magic. That growl & bark they have is SO satisfying! A buddy of mine has a 100 watt Plexi, and I've been trying to get him to come check out how good the Axe III replicates the tones he uses. He uses a Power soak live, and only a couple Boss pedals. He just doesn't know what he's missing. But, some people are content to just have a great tube amp.
Funny thing though, a few years back I played thru his rig, and it was lifeless. I told him it needed new tubes, but didn't agree. (This is a guy who hardly ever even changes his strings!) Anyway, he calls me one day to tell me his amp was acting up so he took it to a tech, and now it sounds better than ever! All the guy did was change his tubes!