Best 40 to 50k used speakers

There are way to many variables to give you the answer you seek.

What are the dimensions of your listening room?
Is it dedicated for audio or multi-purposed?
What are your listening tastes?
What is your upstream equipment?
What speaker performance qualities are important to you?
What speaker aesthetics are important to you?
And as Yul Brynner said, "Etcetera, etcetera, etcetera."

@thieliste

FYI on Alsyvox - https://www.usaudiomart.com/details/649661765-alsyvox-tintorettos/ (though a dealer ad)

My second fav speaker is the Thiel CS3.7, which you seem to be moving away from. What an incredible sounding speaker.

My favorite is the KEF Blade and I am looking forward to hearing the KEF Reference and Blades with the new Meta stuff. Could be a good choice coming from your Thiel background.

A Vivid speaker may also be something you like. I love the Vivid lineup too but I think KEF gets you close (or better with the Blade) for less dough. The new Magico A5 is also something I would like to hear, though I do not like Magico as much as Thiel, Vivid, and KEF, I maybe in the minority in that statement.

The Yamaha NS500, though classified as a bookshelf plays well in big spaces.

Everything I listed comes from big companies who may give you the benefit of Economies of Scale in their design and construction of speakers (Vivid maybe from B&W work).

stenheim alumine 5 .
AudioArts NYC once had one for sale on AudioGon.

This is not quite in the price class, but I've read good reviews of these speakers and amps: http://sanderssoundsystems.com/products/price-list

SO, do we assume a 40-50k speaker pair will go into a bespoke listening room???

Are you a buyer, or just curious? I have my Vapor Audio Joule White speakers up for sale here.

BTW, I wouldn't respond if I didn't think they were properly classed in that price category. 

I just listened to the Alsyvox I Maryland and thought it was spectacular. It has real bass. At that same location I listened to the GamuT RS3i and the RS5i. I think this is it for me. Next stop Gamut. He will take my TAD ME-1 as a trade + $. I do love the TAD but I think the GamuT is a notch above.

Alsyvox is awsome i heard them at Munich Hifi Deluxe.

I kind of agree with the Vandersteens if you include their amps. :)

https://www.audiogon.com/listings/lisa5g62-alsyvox-tintoretto-full-range

You may want to audition it at MD.

Speakers that come to mind :
Verity Sarastro llsJoseph audio PearlWilson alexia 2Magico A5Borresen 03Tidal Contriva G2YG Hailey 2.2...

You can find a pair on here Agon,  for 10K cheaper through legendary audio pro. I got my Dutch and Dutch from them. 

Never heard the Kii 3, are these very good for the price ?

Verity Sarastro lls paired with latest Spectral pre-power ?

Kii 3 with BXT 

https://almaaudio.com/products/kii-three-bxt-complete-system?variant=29324985073717&currency=USD...

Choose the speakers first obviously.I have the choice of getting brand new speakers up to msrp 50k or get much higher end speakers on the used market for the same budget.

Anything from Vandersteen, and definitely under you budget.
And, if you like Thiel, you're probably going to like Vandy's.
You can use the money you haven't spent on great cables, like the new AQ William Tell and Hurricane.
Of course, you need suitable equipment upstream.
Do you have any components, or are you going to buy them when you choose a speaker (which is a good move)?
Bob