I disagree with some of the opinions here about the LS25MKII. I demo'd an LS 25MKII and did not like it at all...it sounds very transistor. I purchased a LS26...but would have purchased a LS17 if my budget wouldn't have allowed the LS26. I've heard the LS17 and it provides 70% of the LS26 performance. In my opinion, the LS 25MKII was not even in the ballpark of performance with the newer ARC preamps (in my system), including the LS17.
However, the best judge of all: your own ears. Try to demo both. I demo'd the LS25MKII less than a year ago and there should be a fair amount of these still at dealers. Call around.