putting speaker vertical or horizontal?

Pabelson, most center channels of H.T. systems are placed 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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Yea most are placed horizontal for space reasons, not because it is ideal

True audiophiles should not use horizontal center channels, even MTM arrays, for music systems. For the reasons why, see Tom Nousaine's article in the Dec/Jan issue of The Sensible Sound (not available online) or on the Audio Critic webzine (subscription required):

http://theaudiocritic.com/blog/index.php?op=ViewArticle&articleId=12&blogId=1

So, sometimes it pays to look at NHT's web-site. The M5 is a center channel speaker and should be placed horizontally.

Regards, Rich