Speakers known for great midrange.

tbh lots lots of speakers get the midrange right...it is more about how the midrange is portrayed in relation to the rest of the music

from quads to spendors to ls3/5a's to dahlquists to vandersteens to harbeths to joseph audio to fritz to proac to atc to audio note to maggies to veritys .... you get the point...

Best mid-range I ever heard was the old model Alon Phalanx

The Alón Phalanx/Poseidon Loudspeaker System is to this day the best mid-range sound I ever heard.  I also jut heard them recently, why I didn't buy them I don't know.

Happy Listening/

I personally own INT-25 paired with Sonus Faber Olympica 2. I'm very pleased with the music filling my room. I would characterize the sound as lush, inviting, engaging, very transparent, relaxed, airy.  I prefer the original Olympica series as the new one I wont say sound brighter but colder to my ears.   

Arguably one of the best mid-ranges is found in the Vandersteen KENTO and Model 7.  Unfortunately, these are floor-standing speakers so the monitor requirement does not apply.  If it were ins a monitor it would be expensive. https://youtu.be/T2pvz6RDBCE 

I’m thinking about going with the CSS Audio 1TDX prebuilt stand mounts. I haven’t seen a bad review of these yet. Or I might spring for the 2TDX towers.