I do find this site useful to visualize mic patterns: http://www.sengpielaudio.com/Visualization-ORTF-E.htm.
Great site and a very useful tool in understanding all this stuff. Okay, so using that visualizer, the grey line which surrounds the microphone pair is the overall combined pattern I was talking about above. Play around with switching the microphone patterns and angles and look at how doing that affects the shape of that combined pickup pattern.
I also get a SBD recording which I mix in post for a matrix mix. The SBD is the PA mix, so my desire is to have the AUD be (somewhat) independent of the SBD - does that makes sense??
Totally.
So in that case, what you want most in your AUD is all the stuff you'd like to have, but aren't getting from the direct, clear SBD. That's mostly likely these kind of things:
1) instrumentation which isn't well represented or present at all in the PA.
2) a good sense of stereo width, depth and image placement of the instruments on stage
3) good audience enthusiasm
4) good ambient room sound
Although you can't avoid it, you really don't need any PA at all in your AUD.
Sure, a typical stereo pair in the middle of the room works fine combined with the SBD, but it may work better to do something that gets you more of whichever of those 4 things you'd like more of, by using a setup or locations which focuses less on the PA. You've done that by moving closer.
Your closer position is working like a stereo pair of mics placed at the stagelip on on-stage. If you can do that, try it. Its usually easier to manage than 12' from the stage in the middle of the audience, especially if they are rowdy. That placement immediately gets you part 1- the on-stage sound with the instrumentation missing in the PA. With a good stereo configuration, that position also gets you part 2. Often you get enough 3) and 4) without doing anything else. If the PA is well mixed and has most everything you need, you can also get away with more odd seeming setups in combination with the SBD like a wide pair at the stage facing the audience for reaction and room, or just wide spaced room mics in back. In those cases the SBD provides all the clarity and solid center in the stereo image, and the wide placement of the ambience mics provides the 'air' and wide stereo feel. Both parts support each other and wouldn't work well alone.
When mixing with the SBD, there is really not much use for an X/Y AUD. The SBD will be mostly center heavy mono, so it's better to err on the side of over-wide with your AUD pair, with plenty of decorellation between channels. The SBD takes care of any fear of 'too wide' or hole-in-the-middle. And a wide spaced pair at stagelip, can cover the stage sources more equally if they aren't all arranged perfectly for you.