Wayne Parham Messages: 18835 Registered: January 2001
Illuminati (33rd Degree)
He was trying to address vertical modes by staggering the height of that one sub. Problem is, vertical modes are higher in frequency unless the ceiling is unusually high. And since you cannot high-pass distant subwoofers high enough to smooth those frequencies without making them sound unnatural, you might try a modified arrangement, leaving the third sub on the floor. It's probably more convenient anyway.
You could place the third subwoofer somewhere "random" but other than a corner or wall midpoint. While that's not Geddes specific placement instruction, I do believe it captures the spirit of what he was trying to do, which was one corner, one midpoint and one "random" placement.