Speakers for string Quartets

Couple of things: If you're interested in British BBC-heritage monitors, do read Robert E Greene's reviews at TAS online. He is a violinist and has reviewed many of those speakers. That could help you choose among them.

I don't disagree with @mahgister that acoustics of the room are important. That wasn't your question, but it does pay dividends.

Listen to some sax quartet music as well

I had Quad ESL 57's for 20 or more years and about 4 years ago switched to Harbeth 40.1's. Never looked back. The Harbeth's are very transparent and neutral and having a lot of amp power certainly helps.

I have the Graham Audio LS 5/9 and they are really great with string quartets, also with Jazz they are sublime.

If in a big room, they need a strong amp, meaning a good power supply.

Also I find that they sound great on low volume.

Good luck with your search.

Tannoy Prestige. Absolutely loved the tone, body, and texture of string instruments and piano I got from my Turnberry SE, especially after replacing the crossover parts with high quality stuff. Surpassed the Sonus Faber Cremona M they replaced by a pretty wide margin in those aspects. Also the scale!

They are pretty high sensitivity and easy load so your amps should have no problem driving them.