Relatively speaking, RP 600M Mk I has certain limitations despite of rave reviews from HR. The high ends are bright, edgy and forward and the bass is a bit bloated. The unbalanced, overly presence of highs and lows covers up the midrange. Fatique sets in pretty easily after a short time period of listening. On contrary, Mk II is much improved. Tonal balance is right, although the treble is still on a forward side but not offensive. Bass is a bit lighter but also cleaner and, hence, Mk II may need a sub. for movie watching. I would say the Mk II is worth the increase in price IF you like that type of sound signature.
It's almost as if someone at Klipsch watched Danny Ritchie's video for his upgrade kit for the MKI,
They literally corrected everything Danny found wrong with the MKI, for their MKII iteration.