What does one purchase after owning horns?

If you could go up a bit in budget the Audiokinesis Dream Makers would be worth looking into. If you prefer something more conventional then the Jazz Modules might work for you.

Large active ATC's or PMC's used will do the trick of integrating bass and sounding dynamic and articulate like horns.

As an owner of Tannoy and Soundlab, htey are very different beasts. Both have the seemless single driver point source sort of sound that I adore. They both also have weaknesses.

Tannoy- PRaT and dynamics! Smaller imaging and less resolution (unless perchance you can go way up the chain).

Soundlab:Resoultion and imaging are first class, but I am always fighting for more macro dynamics and swing.

I am very curious about the Audiokinesis line as Duke was a Soundlab distributer.

The best speakers I ever owned for piano, in particular, were my Apogee Full Range Planars. They have the added benefit of providing a true 25Hz to 25kHz response. But they are big (over 6' tall) and require a small power generating station as a source - well, about 200W x 4 of good old-fashioned SS power at a minimum. A used pair of Duettas or Scintillas would be within your budget, are a little smaller and area little (very little) easier on the power requirements as well. Note - these speakers have about the lowest WAF rating of any speaker ever made, exceeding even the Avantegarde in this respect.

The used pair of Magnepan 20.1s will get you in the same ballpark as the Apogees, less the bass response of course. But I don't know if you can do it for $8K. Maybe a pair of 3.7s and a sub?

That's a really tough one as it's difficult to match the speed and dynamic prowess of horns. I struggled with it after having to go to a smaller listening space and knew I'd be limited to more traditional dynamic speakers because of the space limitation (you may not be faced with that). From one horn lovers perspective here are a few of my likes: Ultimately I was very happy with with Coincident speakers (I had SuperEclipse III's for a few years, but his current models are certainly worth hearing as well and have similar qualities). I'm now using AudioMachina and could not be happier, but they'd be beyond the budget you stated. I'd also second the AudioKinesis recommendation - his stuff is brilliant... definitely fast and dynamic, with seamless integration of the lows with his SWARM sub system. Of the planar stuff I've heard I've really liked Roger Sanders speakers on two different occasions hearing them. Daedalus speakers are also brilliant, especially the larger DA-RMa and Ulysses.