Audio Research LS 25 MkII compared to actual AR production ?

Maybe look for a Ref 3 if you want to stay with ARC. I had a LS25 MKII for a week in my system on loan and it didn't do it for me. I preferred my former ModWright SWL 9.0 SE over it. Ended up with a ModWright LS36.5 which is a big step up over the LS25 MKII, at least in my system. In my system I felt the LS25 MKII was a little lean in the bass as well, not horrible but it didn't pull me into the music like the ModWright preamps do. Good luck.

.
As adg101 says,  get a Ref3.
It will rock your world. 
.

 I had an LS25 mk2 for many years and finally upgraded to a Ref 5se. This was a huge jump and improved on every aspect of the LS25 mk2 in a big way. I have tried a lot of ARC preamps over the years but the Ref 5se convinced me I needed to upgrade.

 What power cord are you using? Shunyata (not Venom) or Sane work really well with ARC. You will be amazed at the difference it makes. I went from Kabala to Shunyata  and the bass improved while the system became far more focused.

Hi all and thanls for your forst suggestion.

Today I tried a Shunyata Black Mamba HC on the LS25. Difference between Furutech FP-3TS20 and Shunyata is great, but I'm puzzled about the bass that became maybe too dry (remember with spectral and wilson I *need* body ;-) ). Tomorrow I will continue my listening session.
Shunyata effort on sound is, generally speaking, invasive, and its contribution has to be evaluated carefully, IMHO (I love Shunyata but I'm conscious they are not cables for novices) .

Ref 5se. Good suggestion , thanks @lostbears
Ref3, thaks @mitch4t  and @adg101, my only concern it's -more or less- a preamp with the same age of L25, I'm not sure I want to buy an old equipment more.