As a thought experiment I've wondered about 4 subs in the front 4 corners of a room. This gets you all the same room modes, but with use of the upper front corners lets you sling about more subs without eating up precious room real estate out in the room. Then being the devious and impractical audiophile I am, my mind went immediately to the idea of a stacked column floor to ceiling in the front two corners for maximum sub output capability. Like maybe a solid stack of Dayton Audio 12 inch subs or some such crazy idea. Eventually of course I end up with a Grateful Dead-like wall of sound. But then the Wall of Sound reminds me of Phil Spector sound and I write the whole thing off as a bad idea.
I believe Mark Seaton has setup several rooms with his subs just that way.