I recently read this article "The placement of one or several subwoofers" by Ingvar Öhman first published in "Musik och Ljudteknik", no.1, 1997, Sweden (don't fret its in English)
It was written before the Harman paper on the same subject. The writer favours corner loading duals and front loading multiple subs to minimize phase errors and interference. I tried this setup once and thought it sounded pretty good. It also looks very cool having all that gear up front in an HT. I never tried his suggestion of two subs near the ceiling and two on the floor but his rational makes sense acoustically.
http://www.sonicdesign.se/subplace.html 
