Lots of good (and predictable) advice here. Electrostatics, the English Invasion (Harbeth, Spendor—and don't neglect B&W, DGG's monitor of choice, favored by John Eliot Gardiner); Sonus Faber, Vandersteen. Quad ESLs get mentioned a lot, and I have a friend with a pair. Underwhelming. No one has mentioned Vienna Acoustics, which sound very civilized with chamber music. So let me add my contentious two cents, just to poke a stick in a few eyes (or ears, as the case may be).
First, and despite the conventional wisdom that any good speaker will reproduce any music well, I do think there are brands better suited to String Quartets vs. Heavy Metal. I like Tool better on a friend's Klipsch corner horns than on my own system. Those speakers can dislocate your spine. No one's spine was ever dislocated by a String Quartet.
When I bought my speakers, I auditioned them against Vandersteens and B&W. Both of those contenders seemed to veil the music slightly, and were distinctly disappointing with loud, bass-heavy rock (the B&W 802s less so).
In my home, I've compared my beloved Teslas (yes, that's right, but they were made by Mike Maloney of "Scientific Fidelity" back when Musk was a toddler) against PSB Synchrony Ones, Von Schweikerts, various B&Ws, and the enormous MartinLogan "The Quest." The Teslas always prevailed. Unfortunately, you won't find them; a bad review from Corey Greenberg in Stereophile pretty much killed them. Too bad. They're beautiful to look at, too.
For what it's worth, I play cello in a String Quartet. Granted, the listening experience is different with the instrument cradled in one's body. But, of course, I've had many hours of experience in the audience. The Teslas rule, IMO.
Check out my "virtual system," if you're curious. Besides the photos, there's a long account of my "audiophile journey," to repeat the cliche.