Well, here's a heretical view: less is more.
It's a capella, it ain't _supposed_ to sit in a mix. It _is_ the mix. So, all that follows here is highly opinionated- but this works for me and the folks I work with. Your mileage may vary.
I do a lot of work with a capella quartets, both male and female. The best stuff I've done to date has been to get the vocalists in their accustomed semicircular positioning in a good room, and mic them with a simple stereo pair- MS or Blumlein works best in my experience, although I've used XY and ORTF from time to time with less wonderful results. My favorite is MS for this, but Blumlein will give you a lot of the room if you have the mics and the room for it. And you _want_ a lot of the room: a capella work in a bad room really isn't worth recording, because it'll never sparkle. I don't overdub, I don't track separately, I seldom spot-mic (because the results are always disastrous), and I almost never use any artificial reboing. If the sound is right in the room, all you have to do is Do No Harm, stay out of the way, and print it! That's why I like recording live so much. it's worth seeking out a venue where the magic can happen...
A capella vocal ensemble work is truly magical, because the players need to hear and tune to each other in real time, and the room is an absolutely critical part of the sound. The players have worked for years to "blend" with one another, so *let them*: you don't want to hear N voices, you want to hear one single instrument with some interesting stereo separations and a very complex harmonic structure! Don't try to enforce artificial separation. Give them a place to work their art the way they've rehearsed it and like to hear it, put the mics in the room with them where it sounds the best, and then get out of the way and punch go. All you should need for postprocessing is a miniscule amount of compression, if any.
Sometimes (for certain style of music) minimalism is absolutely the best, and IMNSHO this is one of those cases: it _ain't_ rock and roll. I'm currently scrimping for a Royer ribbon mic for my side mic (to supplement my C3), and for an Amek 9098 dual mic pre, just for doing this sort of work...
For another amazing example of vocal work on the order of Manhattan Transfer, but more out there on the fringe, get the New York Voices eponymous album. In that case, I'm told that they mic'd up smaller subgroups, and you can hear that in the mix- but the sound is wonderful. "Come Home" was a single-take MS recording, I'm told. Tasty, and good enough to put chills up your spine...
http://newyorkvoices.com/