It's funny because I do have an answer... sort of. First of all, it goes without saying that the other components beside speakers are of paramount importance too. I haven't been exposed to lots and lots of audio gear, but of those I've heard the best electronics for string quartet were Accuphase (oh, and an expensive DAC from Luxman).
This being said, the best speakers for string quartet that I've heard are the Harbeth M30.2's. Overall I preferred my Spendor SP2/3R2's so I sold the Harbeths but for string quartets they were kings. My current Martin Logan Impression 11A are also good (when paired with Accuphase) with better detail, extension, effortlessness etc. but not quite as convincing timbre and less tactility.
So, yeah, Harbeth. And another contender might be Sonus Faber.