I wouldn't claim to know the line-up/history well enough to rank the listed models, but my own impression is that somewhere around the introduction of the original LS 25 (which I own), the overall sound of ARC gear got a little leaner. Some might call it more analytical while others would say more neutral, but I do think there was, for better or worse, a subtle shift in the ARC "house sound" that moved further away from the classically romantic character that some associate with tubes. BTW, I don't think that ARC ever had a ton of that, but I do hear even less of it in their more recent models.
Just MHO.
Just MHO.