AMP or Preamp

It seems nobody has anything negative to say about PS Audio, Audio Research and Pass Labs could be a good place to start.  

All three would be good choices.  Wilsons are extremely revealing of upstream components, so I would try several if possible.  

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.