Best 40 to 50k used speakers

A lot. Have in mind Kharma Exquisite, Avalon Eidolon or Gryphon Pantheon. Unbeatables.

Alsyvox at drvinyl.net
you won’t be dissapointed
the best music I have ever heard

Wilson Benesch Cardinals

http://www.theaudiobeat.com/equipment/wilson_benesch_cardinal.htm

Tidal has blown me away at every show I’ve seen them. 
http://www.theaudiobeat.com/equipment/tidal_contriva_diacera_se.htm

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.