Best 40 to 50k used speakers

Another vote for whichever Focal Utopias fit your room.
I have the Stellas myself, the EM bass is just effortless, even in my difficult room (concrete floor, patio windows behind speakers)

Amplification?
Has to be Boulder. Truly a marriage made in heaven. So rich, textured and authoritative. But above all, musical.
This combination is truly on the side of the performer - producer

My vote (purchase) was for Raidho D-4.1s. Last Raidho model with pure Borresen DNA. They are beautiful and sound great with my wildly eclectic musical choices. I have Focal Utopias too, and while the Focal sound is really good, it can’t compete with the Raidho/Borresen speed. 

This is so subjective to ones opinion, no point to it really.  

@jderson No point really with your comment.

If you like the coherence of MBL, try to give a listen to German Physiks speakers.  You can spend (new) $19k or $250k in their range.  Mine are not for sale :-)