Buy an Audio Research SP10 MkII or an SP11 MkII ?

For the sound you're describing, Wendell is correct, the SP-10 is what you want--it has a very good phono stage too. The 11 is a hybrid design, and sounds more neutral but definitely more like solid state than tubes. If you want to spend more, a Jadis JP80MC, particularly an earlier model, would also fill the bill (I assume you want a phono stage along with the line stage in your preamp).

One thing to note is that while the LS5 is a great product-I still like it better than virtually every line stage ARC has made because it has the tube-like qualities they seemed to move away from in the SP 11, SP 15 and LS 2 hybrid designs-and an excellent alternative (with a good phono stage) to the SP-10, it is a balanced only output design, as I recall (Jafox, am I correct in my recollection?) so you'll need a balanced setup, something I couldn't do in my system. There are adapters available from companies like BAT and I'd imagine ARC, but they will to some extent degrade the sound. Of course, you could always replace the amp with the VT150s that ARC came out with around the same time as the LS5, but I didn't get the impression that the original poster wanted to do much changing to the rest of the system.