Rogue Audio Cronus Magnum - Worth spending $700+ for Mk.III version?

MC capability was the main feature, but the stage was supposed to have been improved in general as well.  With that said, I've never really done a back to back with a CMII versus CMIII only using the phonostage to see if you can tell a noticeable difference.
If you don't mind spending the few extra bucks, then I would say certainly the CMIII is best of the generations.

The new IIIs are shipping with KT 88 tubes.  Do NOT get that amp with KT-88s.  

If you can find a nice CM- II with low hour KT-120 tubes go for it.  The CM II is a winner.  

I would somewhat disagree with the above post....I love the CMIII with KT88s, and actually would sometimes run 88s in CMIIIs even when they were shipping with 120s.  I have several customers that run both, and both have their positives.