I just came upon this post, so I am late to the party. I am utterly devoted to string quartet music, and it has always been a significant factor in my selection of equipment. I’ve personally owned Magnepan, Quad ESL 57s, small bookshelf speakers (not out of the BBC mold) and full range floor standers.
I’ve found that speakers that exhibit anything less than superb coherence will be a distraction, if not an annoyance. Pay particular attention to crossover frequencies. Magnepan speakers, for all their virtues, in my experience all exhibit an obvious transition between tweeter and midrange as a violin descends in frequency. This manifests itself both spatially, where the violin moves in space, and qualitatively, where the difference between the sweetness and purity of the ribbon and the lack thereof in the midrange draws attention to itself. I’ve also heard this in some monitors, where beryllium tweeters are used. They are so fast and clear that the descent into the midrange produces a similar lack of coherence.
Pay particular attention to how instruments are presented in space. This is another area where Magnepan speakers have issues that can be exposed by small ensembles and even more so by a single instrument. The Bach sonatas, as example, sound as if they are being played with a 7 foot long violin. Most decent monitors, floor standers, and the Quad ESL 57 will get this right, or at least better, than Maggies. For me at least, it is two strikes and you are out on Maggies for quartets.
Speaking of Quad ESL 57s, I bought a nice pair of fully rebuilt 57s several years ago based on recommendations from several people whose opinions I deeply respect. A couple of those people are professional musicians. Honestly, I was disappointed. I have Atma-Sphere M-60s, which are a preferred amplifier for these speakers, and yet I just couldn’t get them to work for me. Perhaps they are not right for my room. The midrange is very nice, but I could not get beyond the lack of extension at high and low frequencies. In my opinion, you need that extended high frequency to properly capture the strings. Those upper harmonics are really essential. However, if you or anyone else wants to give a pair of 57s a try, I would part with them at reasonable cost.
The best I’ve heard for quartets in my own system is a pair of Coincident Super Victory II speakers with rebuilt crossovers. These feature a ribbon tweeter that crosses over fairly high, a 7" mid, and a 12" woofer that rolls off at about 26 Hz. The speakers are nicely coherent. There are no clues evident what the music transitions between drivers. Instruments (and voices) are presented with realistic spacial clues. In a well treated and well set up room, imaging is superb. I can at times hear the first and second violins as instruments separated in space. This is something I wouldn’t normally hear even in a live venue. BTW, that should always be ones absolute reference. I trust you are a frequent attendee at live chamber.
The largest downside with my Coincident floor standers is that the midrange is not the most refined I have heard. I suspect that a top notch smaller midrange, perhaps a 5" or smaller, would do much better in this regard.
I don’t think that your leaning in the British direction is unreasonable. I think that the ability to audition is going to be very important for you, unless you are willing to go used and buy and sell until you find what you are looking for.
Good thread! Keep us posted.