What about driver directivity? It seems that most drivers become directive when the wavelength becomes less than their diameter. Taken to the extreme, if a line of 8" woofers is playing above 10kHz I would not expect any comb filtering because the drivers would be beaming like spotlights. Hence in general I would expect that if the c-t-c spacing is greater than 1 wavelength but the inter-driver spacing is kept very small you would notice the sweet spot get smaller, and not necessarily any comb filtering.

-Josh Dybnis