You are asking about one of the most intelligently designed and best-sounding speakers ever made, in my opinion.
I had the pleasure of listening to designer Siegfried Linkwitz's personal Beethoven system in his home about a year ago. Startlingly lively and dynamic, extremly low in coloration, very revealing, and stunning on massed orchestral passages as well as hard rock (Siegfried put on some ACDC for my patient teenage son).
I used to own Quad 63's and am somewhat familiar with the Apogees, and would say that the Beethovens share their low level of coloration but can do dynamics on a scale that takes your breath away.
Whether the Beethoven is the only speaker to outperform the Quads and Apogees in openness, naturalness, and overall loveliness of sound on acoustic music, I'm not so sure. I think there may be others that are worthy successors to the Quads and Apogees in these areas, perhaps including the speakers I peddle (Sound Lab full range electrostats). I count numerous former Quad and Apogee owners among my customers. But I know of few speakers that even approach the explosive dynamics of the Beethovens, and none that do so with such low coloration.
I used to own Crosby-modified Quad 63's with Gradient subs, and the three speakers that were in the running to replace them were the Audio Artistry Beethovens, the Magnepan MG-20's, and the Sound Lab Millennium-1's. I chose the latter, and subsequently became a dealer. If I might make a rough analogy, the Beethovens are like sitting on the first few rows at a concert, whereas the Sound Labs are more like sitting halfway back in the concert hall. It's the dynamic scale, finesse, and power of the Beethovens vs. the lushness and inner harmonic nuance of the Sound Labs. Based on the criteria you cited ("openness, naturalness, and overall loveliness of sound"), the big Sound Labs probably should be on your list. That being said, I can't imagine anyone regretting owning the Beethoven system!
Best of luck to you in your quest!