I just bought a red stripe Mark III combo. It´s fully loaded (simulclass, eq, rev, EV speaker, hardwood). It´s in good shape for a 19 year old amp.
It´s the best Mark series amp I have ever played (I have never played a IIC+, but from recordings of that amp I have heard it sounds pretty close).
The Mark III sounds very smooth if the presence is down and very agressive it it is up. More distortion than the Mark IV I used to have, and a warmer, more natural bottom end. And more importantly, it sounds more organic (the Mark IV can sound a bit sterile at times in my opinion).
I used it at band rehersal the other night with my traditional recto cab, and WOW what a tone. It sounded huge and warm, just about perfect.
The only thing I noticed was a bit of a hum on the lead channel. I think it´s caused by the fan. I´m going take it to a tech to check the amp out.
And the weight of course (almost broke my back carrying it).
The Mark III´s have had a reputation as a lesser Mark series amp. I don´t understand why. It´s great!