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.