If you decide to stay with an ARC amp, I would be biased [sic] on picking up an ARC line stage or full function preamp. There's a natural synergy that's hard to describe. As I mentioned above, my set up is all ARC: Ref 5 line stage, VS 115 amp, CD-8 player and PH-8 phono pre. Everything just works very well together.
But to better answer your question, how much do you want to spend and do you run vinyl. If not, you don't need the phono pre. Also, if you are heavy into digital, you may want a DAC.
To illustrate, on the ultra high side there's a used ARC Ref 40 for sale now -- ask price is about $16K (original retail $24K I think). I see some ARC REF 3's coming up now and then for about $5K. The LS 27 has received quite a bit of great press -- some say almost as good as the Ref 5. I think used LS 27s are listing for about $5K.
If you come back with answers to my Qs, I can make some ARC suggestions. I'm sure others will have their own favorites.