Best 40 to 50k used speakers

You might consider purchase of an old pair of YG Anat and upgrade the innards to whatever level of Sonja components you can afford.  Better yet, find a used pair already upgraded, as I did.  You'll get the sound of the current model without the price tag of the current sleeker cabinets. You'll want a passive bass unit even if the used one has active bass.  I have Anats upgraded to the lowest Sonja level and paid less than 1/2 of what you are considering spending. They compare favorably with the delicious current top of the line YG Sonja used by a reviewer buddy of mine, although listening to his does at times produce a bit of tweeter envy.  Although I have all top notch components, I'll never get the full benefit of what these speakers can do.  Make any downstream improvement, the speakers are up to showing it off.  Perhaps most astounding is how versatile the YGs are in terms of what the room size is.  They can "disappear" in even a very modest sized room.  The YG Hailey's have been mentioned.  They are fantastic but you are unlikely to come up with modestly priced used units.  I'm quite certain upgrading the Kipod model to Hailey level isn't possible.  In any event, the bigger YGs are more fantastic and more room-versatile, even if you need a fork lift to move them.

I moved from KEF Blade 2s to a pair of spotless used Sonus Faber Aidas for just a little more than $50K USD, including shipping to the US from the UK. Since you're in the EU, presumably they could be had for less if you can find them.
List price of the Aidas when first introduced in 2012 was $120K and the new Aida 2 are now $130K.

I'm very familiar with the sound of my original Aidas and have heard the Aida 2s demo'd at an open house hosted by a large Seattle dealer. The 2s are excellent speakers, but I would definitely say they are nowhere near $80K better (the differential between what I paid and a pair of new Aida 2s).
Getting into the realm of personal preference, I've heard Wilson Alexia 2s many times (list around $50K new) and I much prefer my Aidas.

Meridian DSP8000SE? Save $$$ - no amp (or cables) necessary.

Wow, what a nice budget, especially, for a used pair of speakers!
You should end up with something extraordinary~

I spent a few minutes with the responses; no surprises there.
All wonderful recommendations...everyone has an opinion.

I wonder where you could go to audition more than one example of something in this caliber?
Traveling can be tricky these days, but I would hope you would want to listen to as many possibilities as you could find? "The journey's the thing"

I didn't see any mention of Gayle Sanders finest effort with his Martin-Logan Statement e2
What a spectacular system! The design, the presence and the sound quality always blew me away!

Arnie Nudell's Infinity IRS-V is another remarkable speaker (once the woofers have been replaced) Paul McGowan at PSAudio can attest to that...

I 'second the motion' regarding the original Nautilus' (I owned the 8th pair made for over 15 years) what a wonderful experience!
However, they require 8-channels of amplification for the pair, so you really will need another large budget for power...

I wonder if 50k would buy a nice pair of Wilson Audio Alexandria's?
That another pretty great speaker~

Remember, regardless what speaker you decide on, you will want multiple subs to go along with it~ regardless. 

Have a great time with this...
Stay well and safe; best regards,
Jim Gray
Jim Gray Designs
jimgraydesigns.com

I'd buy the best used Estelon speaker I could afford, even if I had to wait for newer models to becomes available used.  There's something deeply satisfying in their presentation of music.