Can your 426 do Blumlein? (I can't remember, I haven't used an AKG stereo mic in probably 25 years!) They're good mics, and also usable as a baseball bat! Of course, M/S will usually have better audience noise rejection than Blumlein, and you can modify the M/S image in post production, but I have always preferred Blumlein just because it sounds so pleasing.
Schoeps makes good ribbon mics, but you should also try one of the the two-element Blumlein ribbon mics that I bought from Spieden years ago. It's still available, but I don't have a link at hand. Very reasonably priced, too. Don't place them under any air conditioning ducts.
IMHO, after doing professional Classical recording for a few decades, I'd probably just put out a couple condenser omnis (BK4003 or Sennheiser) spaced about, approximately, somewhere around 2/3 as far apart as they are from the source. Also, I'd move them up and down to establish a good front/rear balance. Don't be concerned if the spacing or height are different for each mic, even if somebody complains about it "looking funny." Of course, spaced omnis aren't coincident, so if you need stereo/monaural compatibility, it's not a good bet. Personally, I prefer the spacious image of the spaced pair, unless I'm recording for broadcast. That's when it's time to go with the Blumlein.
Remember, the room (and audience) sound is an integral part of any concert music recording. You'll never control the recording situation like we can in a studio.