Best 40 to 50k used speakers

Whatever from Tidal you can afford in your budget. Specially if you're from Germany - easy to check out German speakers. 

https://www.tidal-audio.com/

I would also check out Kaiser:

https://www.kaiser-acoustics.com/

Meridian DSP 8000se. 

havoc, 
I sent you a PM.

If you like Thiel speakers try to find used CS6s.  It might be difficult to find something better even if you pay 10 times more. CS 3.7s have nice midrange but the base is incompatible with CS6s which are truly coherent speakers with  very good coaxiales.

It could cost $40-$50K to outdo many great inexpensive speakers such as my Legacy Focuses $2.5-$3K used). However, I am planning to purchase Von Schweikert VR9 MkIIs very slightly less than that. Those were $115K 5 years back. $40 to $50K can get you new VS speakers (or as stated above the VR55 Aktives used) or Lumenwhites which are great. I would have bought the VR55s but they are much less efficient than the VR9s.

Some people prefer Magico (I would never own the Q, S or A series, haven’t heard the M series), Wilson (don’t like the expensive ones), B&W (cheaper new than your used budget and are also power hogs), Focal, Tidal, Giya, Rockport, Vandersteen, Kharma, Avalon, Marten, Evolution Acoustics, etc. all who make and made expensive speakers which can sell for your price range at 40% to 60% less than when new. Bigger Maggies new are also less than your budget and do amazing things.