putting speaker vertical or horizontal?

The usual explanations involving dispersion characteristics don't explain why vertical souns better even when you are listening straight on to the speaker. Here is another theory...

In the typical speaker the Woofer and Tweeter are lined up vertically (when the speaker is vertical) so that the phase relationship between drivers, while it may be imperfect, is the same for your left and right ears (assuming your ears are mounted on the sides of your head). If the speaker is horizontal, one ear is closer to the tweeter than the other ear so that the driver phasing relationship differs between ears, and the brain doesn't like this. So the vertical speaker orientation sounds better. All this assumes that you are sitting up. If you lie down on your side a horizontal speaker might sound better.

This theory would suggest that a MTM driver configuration (a woofer above and another below the tweeter) ought to sound OK when horizontal (neglecting dispersion issues). I do find this to be the case.
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Pabelson...Why don't you just tell us why MTM speakers don't work well horizontal, instead of citing links we can't use?

Kr4...I understand what you say, but it doesn't always seem to work that way. Suppose you had two drivers spaced 6 feet apart...wouldn't they (ideally) image as one driver between them? Now, put the tweeter at that spot. As a point of interest, just about every speaker designed for center use is of the MTM configuration.

Kr4...I absolutely agree that a physical center speaker is better than the best imaging stereo pair. I use a center speaker identical to the front left/right...a MG 1.6 with a subwoofer. Nevertheless, imaging does work. Some people with HT systems use a pair of center speakers flanking their video screen.

Perhaps also if interest...I am old enough to predate stereo, and I remember state of the art systems back then which used two speakers positioned as we do today for stereo. This eliminated the sensation of listening through a hole in the wall.