I had a SLP-05 for almost 2 years I choose it over ARC because ARC sounds too analytical and dry for me. SLP is a great preamp, it has typical Cary house sound - warm, tubish, romantic, not to expense of detail - after replacing stock tubes with some expensive NOS I enjoyed it a lot.Then I realized that it brings some artificial sweetness to the sound. Now I have CAT Renaissance - it is in the same price level, but sonically SLP-05 not even close to it. I think that a lot of audiophiles would agree that CAT is one of the best preamp. It does not have remote control, headphone output, but it gives you true presentation of the music without adding any coloration (BTW for $2000 more you can get version with outstanding phone stage).
Bottom line it is up to your preference: if you really like sound of your LS26 - go for ARC RFF 03 - it is definitely step up, if you want something more sweet and warm go for SLP05.
But I have chosen CAT and very happy with it.
Good luck.