Here is a good idea to investigate:
https://www.high-endaudio.com/used_he.html
Arthur Savadore is probably the most knowadgeable person I know of in this hobby, and he has a set at your price that may fit your needs perfectly. Any time I've tried anything he has suggested, the performance has been 100% spot on.
He considers "sound floor" as his major criteria for evaluating speakers. That sounds like what you are looking for.
Personally, I either would not recommend any suggested above that I have heard, or I am totally unfamiliar with the models suggested. The reason being is that you are currently using electrostatics, and I would think that a different build philosophy may get you where you need to go.
If you happen to be in Florida, Arthur is based out of there.
If you decide to investigate this, tell him "hi" for me, and I haven't forgot about the Ella Fitzgerald review.
Hope it goes well!
Sincerely,
John