I'm a huge fan of Mid/Side. I listen with headphones/earbuds a lot, and generally XY is hard for me to listen to. It sounds distant to me, no sense of space. Mid/Side isn't that way at all. I know there are people who will argue they are mathematically the same, but all I can say is my ears and the listening part of my brain is much happier with mid/side. I've used an LSD2, 414's and ADK's for mid/side, and those same mics + SD mics for XY. Now we have the AT4050ST, which is permanent mid/side, but it outputs a L/R signal as you describe you video recorder doing. I'm constantly amazed how "listenable" those recordings are to me. Generally I'm very happy with that fixed 127 degree setting, but it's easy in post to change width in post if I want to. I do that probably 20% of the time.
I was just listening to SmokinDaughtor Liz's AT4050ST recordings on the LMA last night which you linked to in another thread, Joe- they sound great! I really dig it onstage in the 127 degree setting.
I suspect the difference in sound you ascribe to X/Y verses M/S is due to significant differences in polar patterns and their interaction, which aren’t especially obvious at first glance. Yet after examining things more closely, those differences should be no more surprising than the expectation that cardioids often sound different than a hypercardioids used in an otherwise identical mic arrangement.
When most people think of X/Y, they think of using mics which have a cardioid pickup pattern, yet the virtual microphone patterns we get with practical M/S are never cardioid! OK, I should never say never.. you can arrange things to form cardioid patterns but not in any practical mid/side arrangement anyone around here would use.* Most people record Mid/Side using a cardioid pattern mid microphone, and I assume the AT4050ST is no different. Mid/Side with a cardioid mid always decodes to a crossed pair of mics with some variation of hyper/supercardioid pattern. The shape of that pattern changes depending on the mixing ratio, but it’s never a cardioid.* An equal 50:50 mix ratio of cardioid mid and a fig-8 side produces the equivalent of 127 degree crossed hypercardioids. So the more appropriate comparison would be with X/Y hypercardioids arranged with a 127 angle between them, rather than X/Y cardioids crossed at any angle.
IME, the presence of reverse polarity rear lobes is essential for achieving a good sense of openness and space in a coincident mic arrangement, something which almost always seems lacking with X/Y cardioids to my ear. Fortunately standard Mid/Side using a cardioid mid provides exactly that, but X/Y hypercards would do the same. A supercardioid mid mic can work especially well too IME, producing stereo patterns which are essentially one step closer to Blumlein crossed 8s, but not quite.. and the not quite often works in our favor.
*There are three ways I can think of to get a true cardioid pattern from M/S:
1) Use a cardioid mid and mix in no side at all. The result is two forward facing cardioids with identical information in each channel- dual channel mono. So this one doesn’t really count.
2) Use an omni mid and set the M/S decoding ratio to 50:50, which is the only ratio using an omni which will create cardioid virtual patterns. Note that 50:50 is the only mix ratio that will do so, all other mix ratios will produce various subcardioid or supercardioid patterns from omni to figure 8. The resulting virtual cardioid patterns will face in opposite directions with an angle of 180 degrees between them. Anyone around here do that often?
3) Use a subcardioid mid and set the M/S decoding ratio to the one position which will create virtual cardioid patterns. Again, there is only one mix ratio which produces virtual cardioid patterns, which will be a ratio with more mid than side. The resulting cardioids will have a more reasonable mic angle of less than 180 degrees between them and this arrangement would probably be more useable than an omni mid for our type of recording, but that’s speculation as I’ve never done it or had any motivation to try it (as mentioned, I strongly prefer the sound of some opposite polarity lobe in my coincident setups, and I suspect that's the basis for Joe's preference as well). Anyone around here use a subcardioid mid often? It could be an appropriate choice onstage I suppose, but I’d be more tempted to go in the opposite direction.