slightly off-topic, but if you are just now exploring M/S stereo recording of music, I strongly recommend learning what you can do by equalizing the "S"-channel signal, especially at low frequencies. When you record with a coincident or closely-spaced pair of directional microphones, you're opting for image stability over spaciousness. That usually makes sense, since with widely spaced microphones it's very distracting when a particular sound source seems to jump from one loudspeaker to the other during playback (e.g. a solo soprano with orchestra, if she simply turns her head from side to side a little).
But you shouldn't have to give up all sense of the size of the room; big spaces just feel nice, and that's largely conveyed by the uncorrelated low-frequency information in the recording. Unfortunately since low frequencies have long wavelengths, two coincident microphones will tend to pick up the low frequencies very nearly in mono--all the more so if they're cardioids and/or their polar pattern broadens out at low frequencies and/or the angle between them is too narrow, as it very often is. But with M/S recording you actually have the difference information right there as one of your microphone signals (the "S" channel), and you can boost the low frequencies (room noise permitting) to recover some of the benefit that a more widely spaced pair, and/or a pair with better low-frequency directivity, could have given you.
That has its limits, and sometimes you want to make a corresponding, or partly corresponding, bass rolloff in the "M" channel, but it can really make the sound more pleasant and fun, and it's not necessarily dishonest on a technical level.