Advanced M/S tip for stage-lip or on-stage:
The sound sources on stage can be lopsided or otherwise off-balanced in level, tone, and/or spatial distribution due to the arrangement of players, gear and other stuff. Of course it's always best to accommodate that as best as possible with your recording position, but here's a trick which can provide some welcome degree of control after recording- You don't have to use the same M/S ratio for Left and Right channels. It may be advantageous to experiment with different M/S ratios for each side if you are decoding the raw M/S later and are taking the time to try and dial everything in optimally. Using a different ratio on one side verses the other will vary both the derived polar pattern and mic angle on that side verses the other.
Probably misreading, what you intended, Gutbaucket, but I find this post confusing as it seems to imply that one can use a chosen M/S ratio on a _single_ channel?, You need two signal channels - the M and the S <grin> - to apply M/S sum & diff. matrixing on. Also, changing an M/S ratio can't correct a signal that's "lopsided" in the left vs. right sense. For such a correction, you'd just tweak in post the relative levels of the X(left) or Y(right) decoded signals.
As you said, varying the M/S ratio certainly changes soundstage width and perspective. For illustration, idealized mic patterns assumed, combining a cardioid M mic [polar pattern: "V = 0.5 + 0.5cos(theta)"] with a Fig-8 S mic ["0.0 + 1.0cos(theta)"] yields the following scenario's:
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M level => virtual XY pair
(rel.to S) included angle mic pattern
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+6dB => 90.1 deg 0.414 + 0.586cos(theta)
+3dB => 109.5 deg 0.366 + 0.634cos(theta)
0dB => 126.9 deg 0.309 + 0.691cos(theta) <- supercardioid
-3dB => 141.0 deg 0.250 + 0.751cos(theta) <- hypercardioid
-6dB => 151.9 deg 0.196 + 0.804cos(theta)
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If a recording made with on-the-fly decoding to XY turned out to have had an inappropriate M/S ratio, then it's easy to adjust this in the DAW in post via a double sum & diff. matrixing step:
mic input MS -> XY in DAW ->(rebalance L, R)-> X'Y' ->(sum & diff.)-> M'S' ->(rebalance M, S)-> M"S" ->(sum & diff.)-> X"Y"
Again, I'm sure you know all this stuff, so apologies if I've missed the point..