get as close to the PA system as possible
Not sure if I agree with that. I've taped a lot of comedy and I've got the best results by simply keeping my distance from the crowd. Of course, it helps if you're also near a speaker, and if the place is packed full then yes, you definitely want to be close to a speaker.
Best show I taped was Doug Stanhope at the Baltimore Improv, it was a late show on a friday night and the place was barely half full, so I took a seat in the back, had a speaker pointed right at me, got just the right amount of crowd noise.
A lot depends on the person using the microphone. Some comics are really good at holding the mic steady and keeping their voice at a constant volume, but other comics can be all over the place. I taped Joe Rogan (yeah, the Fear Factor guy) and he's probably the worst I've seen when it comes to that. He would speak very softly at times and then all of a sudden yell really loud, without bothering to hold the mic away, so in playback I have to turn the volume up just to hear him, then kill it real quick when he starts yelling. If you're next to the PA with that kind of comic you'll probably want to keep a close eye on your levels.
Glad to see this thread, and I can't believe I missed that other one. I was beginning to feel like the only person who tapes comedy.