ARC LS 25MKl vs LS 25MKll....?????

SAME STUFF, DIFFERNT DAY!!!...especially when you consider diffent tube options!
Really, if you're considering SWEATTING over debating between the two, It's a SIDEWAYS move at best! If you have the MK I I wouldn't sell it for MK II!! There's going to be little if no change at all overall! If you want a change in sonics, try another preamp entirely. or at least get better tubes than stock and experiment. INfact, call Kevin Deal at Upscale audio in Upland, CA and ask for advice. That's my suggestion, as one who's sold AR for years!
Also, the MK 25 isn't any different sonically from the LS16 even! It's just got more in's and out's and such. (That was straight from the tech's I know at AR!!!).
You'll have to move up to a used Ref 1 or 2 to get improvement of significance in the AR line. Or, find an old used Copeland, CAT, or other better sounding pre's! (The AR's ad a bit of warm bass/lower mid-range to neutrality for my traste) Good luck

I don't know who was telling you that, but I must respectfully disagree. I have basically the same set up as you. I started off with the MKI and had it upgraded to the MKII. After the upgrade, it is MUCH more open sounding.

Foreverhifi;
You are quite misinformed. I upgraded from an ARC LS2B MKI to a MKII and found the improvements significant in musical terms. The MKII was less mechanical sounding, more transparent and refined with enhanced musicality. Also, I suppose the "MK25" that Foreverhifi alluded to is the ARC LS25 MKI. The LS25 MKI is an excellent preamp, especially if used with NOS 6H23 tubes. Sometimes "a little knowledge can be a very dangerous thing." (I own both the LS2B MKII and the LS25 MKI preamps.)