Hi Duckman,
Yes, I mentioned that running an identical pair of speakers outboard of your main pair--in other words, an additional left speaker positioned farther to the left, and an additional right farther to the right of your main right speaker--greatly widens the soundstage and seems to make it more seamless. Call them "outriders" if you will.
We (Floyd Toole, myself, several others on the NRC listening panel) did this in a comparison of very early "bucket-brigade" surround processors, and we discovered that simply adding an extra pair of outboard speakers at the front provided as much realism as the special surround processors with additional surround speakers.
It's interesting that this principle has been applied by Yamaha in their "presence" speakers and also Dolby in the "height" speakers, which really just add more ambience.
I never explored stacking speakers. I wouldn't worry about comb filtering effects. It's a measurement artifact and the tests we did at Axiom suggest that the ear/brain actually likes to hear comb filtering effects.
Alan
