Which Audio Research Preamp?

I currently have an Audio Research SP-9 MKII driving an Audio Research Classic 60 which in turn drive Vandersteen 2Ci's. I've been thinking of upgrading the preamp and have favored an Audio Research preamp, preferably one with a phono section. After reading many reviews and watching the classifieds on Audiogon there are a couple that are full featured and worth considering, the SP-14 and SP-15 and quite a few that are just a line stage (and require me to buy a phono preamp); the LS-5, LS-15, LS-16, LS-22, to name a few of the newer models in my price range. The question is are all these worth serious consideration and would they be a significant improvement over the SP-9? If anyone has recommendation for other brands that work well with the Classic 60 I'd be interested in hearing those too. Thanks!