Why not use mains as centers

Kal,

You are quite correct. Speaker arrays (same drivers outputting same frequencies) are best avoided except in the bottom two octaves (assuming one cherishes precision). You get lobes in response as well as comb filtering of certain frequencies which makes the audible response much more variable than a point source.

As I stated earlier, the issue is the relationship of the driver spacing to the wavelength. In the bottom end, the wavelengths are large enough that the relative placement of the drivers is inconsequential. I was surprised at the resistance to this issue here as I thought it was a fairly well-known matter.

Kal

All I know, is that where I have set it up, it sounded fantastic. Not sure of the physics behind it, but the 2 MMG's emitting the center works very well in a home theater I set up for someone in my neighborhood, who didn't want to give up his MMG's.

Good. As I said, and for many reasons, Maggies are a special case.

Kal

Do people really think the extra effort put into time aligning the drivers, making non-parallel cabinet faces, controlling horizontal dispersion, etc. becomes unimportant in the most important sepaker in the system? You can either think it is unimportant in all of the speakers, or important in all of the speakers, but it doesn't make sense to think it is only important in the side channels, but not in the center. In both stereo and multi-channel, you are trying to get image continuity between an array of speakers - same problems whether you have 2, 3, or 5 speakers.