Seeing as you love the LS50s, you could try
- Feed the KEFs with a tube amp
- Try a tube preamp-e.g, Schitt Freya Tube Preamp, removing the NAD (under your budget)
- Add a high quality, musical subwoofer
Each of these has some big benefits and would result a potential meaningful change without ditching the KEFs.
If you want to replace the KEFs, there are great example suggestions above. I loved the LS50s, but I preferred the Wharfedale Linton Heritage more--yet it is undisputed the KEFs had more detail and were a more advanced design. The Wharfedale are super musical and smooth.
I love the suggestion on Heresy. If you could swing it a used Forte III (likely in the $2K range, they would sound super in that room size and you might not ever want a sub. Heresy IVs are supposed to be spectacular. I haven't heard those. Disclaimer: I moved into sampling and keeping horns about 3-4 years ago. So I like them. The power and clarity (AT LOW VOLUMES) with that Schitt into a Heresy or Forte would show you a relaxed big audio picture. They are so efficient that things should open up to your ear.
Enjoy your journey! You are in a fun spot. My recommendation is to have some fun if you have the time and patience.
Sometimes we just need to try things out for ourselves. It can be super fun, but also stressful so keep us posted!