15k USD Speakers for Classical and Pop Extended Listening, Near setup, Low volume

Low volume listening favors high efficiency drivers.

Check the newest Tocaro 48. Its a 15" full range driver. Magic. The 40 would work well too.
If you are into music but not into hifi, checkout Tocaro (and Crimson)

Wow! Nice size room for an office. I am a biased Maggie fan. So this will come as no surprise. The LRS + set and a Rel 8” sub will get you the biggest soundstage and dynamics at anywhere near its price point. That would open up your budget for electronic devices such as streamer/phono, DAC (if digital) and a high powered high current amp. Just my two cents. 

Sonus faber Maxima Amator. Right in your price range and ticks all your sound quality/characteristic boxes. 

Recognize I speak from personal preference and you should purchase based on your preferences and based on audition. I find KEF a little aggressive on top. I find the ATC could be a little more rich in timbre but overall a great performer.  I never heard the Monitor.  As I have in past posts I recommend you audition Vivid. The models I recommend are the Kaya 90 and better the Giya G3. I chose the latter.  They well perform like the ATC but have a rich timbre.  You will need to get past their unique aesthetic.  I love the look.  You will have to go preowned to be at your price point. They come up occasionally on this and the other site.  My room leaves much to be desired due to the WAF.  They still sound coherent, balanced, with a wide and deep stage.  I sit about 7 feet from the speakers and they are extremely coherent and balanced with bass better than ATC and one of the best high ends.  I recommend after market vibration products of your choice to extract full potential.  Read pro reviews and more important, audition.  Listen to how natural cymbals (the hardest instrument to get timbre correct) are reproduced.  Let your own ears/mind make the choice.  Good luck

No recommendations about speakers from me on this one, but just a remark about the poster advocating for placing the speakers BEHIND the listener. I could not disagree more. As far as I know the human ear is designed with the full opening in FRONT of the ear and a flap in the back to better direct the sound into the ear. Just saying, why fight mother nature on this one.