Best 40 to 50k used speakers

Assuming the $40K - $50K is your assumed new speaker price, I went through a similar exercise a year or so back. Also, we have about the same size rooms. My top contenders in no particular order were:
- Vandersteen 7 Mk II
- Vivid Audio Giya G2 S2
- Evolution Acoustics MM3
- Wilson Audio Alexia Mk II
- Magico S3 Mk II

After hearing all above I purchased the Giyas which, to my ears were the most cohesive (blending of drivers), effortless at any level, accurate (not warm or sterile) of the bunch. I may have bought the Vandys if I could have found a pair used. I think I liked the Vandys highs better than the Vivids but the Vivids play louder more effortlessly and cleaner than the Vandys. YMMV

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.