Well I got a certain (very early) mid 90's Mark IVb that has more mojo and clarity to it than any other Mark I've owned, and it sounds and feels different than the later B revision Mark IV's I've tried. I've A/B'ed it with my old III red stripe and as soon as I did that I sold the red stripe, it just lacked the low end of the IV, it lacked the grind, it lacked the clarity and it was hard to dial the mid honk out of the III, as well as the harsh top end... The III's don't record nearly as well as the IV's do either IMO.
Don't get me wrong though, I still like the III's, but if I had a choice of a III or a IV I'd take the IV any day of the week. The fact that it is really hard to get a modern tone out of a III is kinda a deal breaker for me too.
I've only got to try a IIC+ once and it did sound great. It certainly sounded like an expensive amp and felt amazing to play through, but I didn't play it nearly long enough to judge it completely. IMHO though, I really don't think any amp is really worth that kinda cash.