Differences btween ARC preamp ref models?

ARC comes in 4 dif models ref1 mk1 and 2.ref 2 mk 1 and mk2? (plus new ref3 not of intrest because of price). Whats the difference btween this models. Is the latest ref 2 mk 2 the best? I will use it with Krell monos supposed to be a good match. Whats a fair price for used ref 2 mk2 and for older versions if they are better?

Thanks in advance

Ulf from Sweden