Originally Posted by boojum
chamber quartets were not written to be heard up close and personal.
Actually, they were written only for close and personal use. That's why it's called "chamber" music. Most of this was not even written for an audience, but rather for (and sometimes by) the musicians themselves. This is the Haydn/Mozart era. Moving on through music history we can see chamber music having a listen-only audience too, but never was this type of music intended to be played in concert halls seating 2,000 people.
Originally Posted by Boogaju
If your goal is to add a bit of detail, attack and presence to a main AB omni pair, then do spot each instrument. It will give you the best flexibility in the mix. Though I don't particularly love them, the KM140 (or 184) are great for exactly this role.
Your setup will be a bit more complicated (extra mics, extra stands...) and you might end up using only a tiny amount of each spot mic in your final mix, but it should work well.
This is the safe way to do it. You don't need to use all the spots in the mix, but rather place them than miss them. Focus on getting a good balance (wet/dry and ensemble-wise) from the main pair. Spots can give you presence, clarity, attack, or they might just feed a reverb if you feel someone needs a bit more than what's there.
Any half decent SDC will do, however for strings I tend to prefer Neumann 184 and Schoeps MK4 over my Oktava 012s (go-to brass spots, though!)