First question for M/S success. What does the room sound like? This in a guitar will be the main feature. In fairness, if the room is gorgeous sounding, or the room is totally dead, then M/S lets you set the width of the sound source afterwards, but in a guitar, what actual sound spread do you really have? Tone from the sound body end, then the hole area then the sratchy stuff from the fingerboard. Do you want to really add that murkey sound and the tinny sound to the central darker area? It would record well, but through speakers even in nearfield, the guitar would sound pretty big, so you'd lower the side mics making it more mono, so what would the point be? Real piano, harps, even drums, that have at least speaker width dimensions in real life work well and it's quite realistic, but M/S on smaller instruments is a sort of audio magnifying glass. Record big things and it's a great tool, but for a typical guitar - which isn't really and instrument with real width, just different tones from different parts, why not just two mics, add a tiny bit of pan and blend the two together. Most real stereo techniques go very strange when used close in. I saw somebody recording a violin the other day with a close X/Y pair. About a foot from the violin. When the player stood ready to start, one mic was pointing to his chest, the other pointing to a wall. Then he started, and he wasn't a really wave around player, but the left mic went from his boddy to the bridge area, the right from the wall to the bridge area. I smiled thinking that in the mix, both mics would have to be centred on the pans or the listeners would have been sick, especially on headphones. Guitars are not as bad because they don't move that much, so two close mics in X/Y just act as two mics with different aiming points - not as 'stereo' mics. On a forum people were discussing X/Y or ORTF as a close mic technique, missing the entire point that close in - they are just two mics. All that amazing stereo science going right out of the window. How many drum kits have you miked up where by accident, two mics are at 90 degrees to each other, with their ends closeish? Loads, I bet - they're not stereo mics are they?