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.
