And -- I've NEVER heard an above-the-screen / below-the-screen setup work. Not only are the centers too far apart to integrate, but the different refection patterns from the floor and ceiling keep the sources "psychoacoustically distinct".
Just curious if this is actual listening tests or just hearsay?
Edit: Nevermind, you have never heard that setup.
I've heard this setup many times in venues ranging from living rooms to home theaters to commercial listening rooms. And I've tried it many times myself, with different centers at different heights off the floor.
An above-below center setup is usually a subjective improvement over a single center setup when using 5.x" or 6.x" woofer centers in larger rooms and mid sized theater rooms, but does suffer from the problems I mentioned above (its all about tradeoffs).
