Mark, on your question about a sub exactly in the middle of the room, the benefit is that there's the point where both the modes for length and width are at a minimum(to also get to the minimum for the height mode, the sub should be raised half way to the ceiling). The disadvantage is that the lack of reinforcement from the room would lower the sub output. The Harman paper which I mentioned and Rick linked studies a lot of possible locations. Middle of opposite side walls, for example, is at the minimum for the length mode and although at the maximum for the width mode, they're of opposite phase and tend to cancel. Diagonally opposite corners are at the maximum spots for both length and width modes, but being of opposite phase tend to cancel those modes.
