Here's the basic gist:
In less than ideal acoustic situations, it pays to make efforts toward getting the clearest recording of the PA as possible. Good clarity is more important than a nice wide stereo image. Easiest way to do that is get closer, and point a pair of directional mics directly at the PA speakers. When you do that you'll find that in many situations the resulting angle between the microphones isn't very large, often less than 90 degrees. So in the interest of making an even better recording with decent stereo qualities in addition to good clarity, it helps to use more spacing between microphones to compensate for the overly narrow angle between them. The whole improved PAS thing is all about determining the most appropriate spacing based on whatever the angle between mics ends up being when you point them at the PA speakers. As the angle becomes narrower, the spacing becomes greater, and vice-versa. That increased spacing helps to improve the stereo qualities. It can also help reduce problematic phase interaction problems. It will at least shift the comb filtering around in frequency, possibly making it less audibly problematic.
X/Y takes that the opposite direction. Because there is less spacing between microphones (none at all), phase differences between channels are minimized. Ideally you would want to use increased angle between the microphones to retain good stereo qualities, but that may point the microphones well outside the PA making clarity suffer. However, if you find that a coincident X/Y configuration solves the phase interaction problems that may be more important to you than getting a wider stereo image. If you are okay with the more narrow/monophonic stereo quality that results from X/Y using a relatively narrow angle needed to keep the mics pointed toward the PA so as to retain clarity, no problem. You might then play with how wide you can angle the mics while still retaining sufficient clarity from the PA.