AMP or Preamp

I really like the way my Wilsons sound using Atma-Sphere MA-1 mono blocks which I found used, in your price range. I have also heard Wilsons powered by ARC Ref 150 and that also was a great match.

I do not own Sashas so if you do decide to look at used Atma-Sphere amps I would contact Ralph directly ((651) 690-2246) to get his opinion concerning the possible impedance issue. He will have a solution if there needs to be one.

I also agree with an earlier poster that said almarg and atmasphere are very good resources on these boards.

It seems like several of ARC's components match well with Wilson's. 

You are living one of my audiophile fantasies, here's what I would do.  First make sure you have basics covered - decent cabling, power conditioning, room treatment - if you're starting from scratch this could be $1000-$2000, but all must dos to get the most out of your kit.  Next, and again this is me, I'd get a Schiit Freya (<$1000), to be able to have an ok pre and be able to compare tubes vs ss in this stage with everything you'll be trying next.  Now you've got $3-5000 left and the fun begins - start cycling through amps.  If you're close to a solid dealer demos might be an option, Music Direct and others have 60 day 100% customer satisfaction guarantees, and then there's the whole used market.  All require some work and cost (shipping for example) but you can try out several incredible amps in your room over the course of a a few months or more.  By the time find the amp of your dreams, you'll be ready to reconsider the PRE or maybe add the analog source, or maybe you'll get tweaky with cables or isolation or tube rolling.  Tons of good suggestions on amps above, I would +1 BAT, I've heard Wilsons with BAT and they sound splendid.

How about a Don Sachs DS2 pre, with a whole slew of eligible amps?

Thought I'd share my experience, as I upgraded a few months ago from Wilson Sabrina to a used pair of Sasha 2.  I've been driving them with an ARC Ref Phono 2SE, ARC Ref6 preamp and a D'Agostino Classic Stereo.  Truly fantastic combination, and the Sasha's sound wonderful with that.

BUT - I have a small setup in my home office with a Cary CAD300SEI integrated amp.  This is an all tube SET integrated (Single Ended Triode, if you're unfamiliar with tube amps) based on the 300B tube.  SET amps are pretty magical for those that appreciate what they offer, which is an absolutely glorious midrange.  I had some buddies over, and decided to hook the Cary up to the Sashas.  All 15 watts of it!  Yes, the Sasha's have a pretty challenging load (4 Ohm nominal with a dip to 2.17 Ohms at 90 Hz), though they're pretty sensitive (92 dB).  I have to say, the Cary/Sasha combination sounded fantastic as well.  In my particular setup (that is, a very lively room with hardwood floors and no treatments beyond rugs, upholstered furniture and drapes over windows) neither I nor my friends felt anything was lacking in terms of available dynamic range, and that midrange magic was still there.

To me the bottom line is to listen to as many combinations as you can.  There are a LOT of great choices out there, and you should give as many of them a try as you can.  Take your time and enjoy the process (I rather miss the large amount of time I spent in dealer showrooms listening to different combinations of gear).

I'd also agree with the advice to not worry about a preamp or integrated amp with an on board phono preamp.  For your setup, if you're serious about vinyl you'll eventually want a dedicated phono preamp.  If you're just starting out there are a lot of good phono preamps for under $2K - or even under $1K you could get just to get started and upgrade later.