Just had a thought..
Let's turn this whole thing on it's head.
Could I mic in X/Y and use the M/S function in my preamp or recorder to record the M/S signals?
Assumming for a moment that the extra matrix steps DO deteriorate the sound appreciably, I could then adjust in post without the need for extra matrixing (if that matters at all
, it's probably splitting hairs),
More importantly, this could open up some interesting mic techniques.. like choosing polar patterns and mic angles based on other issues than the resulting stereo sound stage.
For instance, if using mics with less than stellar off-axis respose in a 120deg. X/Y setup, I could reduce the angle between mics for more tonal accuracy towards the center, then dial back in more stereo separation by increasing the S signal ratio later to compensate.
Unfortunatley I don't think the matrix built-in to preamps and recorders will work 'in-reverse'.