putting speaker vertical or horizontal?

I have a NHT M6 speaker. Any difference in sound by putting speaker vertical or horizontal ?
Never put speakers horizontal, unless they're designed to be put horizontal.

And don't buy speakers that are designed to be horizontal, either.
Horizontal is no good, it wont produce the stage well and you will have alot of vertical wave constraints.
NHT's speakers are designed to work in the vertical position. The placement of the woofers and tweeters on the speaker baffle does dictate how/when/where the sound will reach you. Placing the speaker on its side does change that. If you care to search through Audio Asylum, there are threads about how the D'Appolito array (typical horizontal center channel speaker) is not all it is cracked up to be.

If your room set-up demands a speaker to be placed on it side and it sounds OK though, why should you care? If the option is no music in the room ... you can live without the entire audiophile creed intact. This a hobby that does welcome experimentation. For a while though, I did use my NHT SB2's on their sides and quite frankly, they sounded pretty good.

Regards, Rich
I remember I had a pair of AR-12's, and they even had the logo on a post on the front, so you could swivel it for horizontal operation. I tried it once. Just once. They sounded awful.
I'm with Pabelson on this one.

The M6 should be better vertical. Try them both ways and see if you can tell the difference.
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.
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):

Pabelson...Why don't you just tell us why MTM speakers don't work well horizontal, instead of citing links we can't use?
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
Horizontally-arrayed drivers which are farther from each other than 1/2 wavelength, based on frequencies which both play (for example, at crossover), will result in a pattern of cancellations and reinforcements in the horizontal plane. This results in varying frequency response as one moves even slightly off dead-center. Besides, both ears cannot be dead center at the same time, anyway.

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.
Would they image? Possibly or we would not have stereo, would we? OTOH, that is with slightly different signals. If you compare a mono signal over the pair with a mono signal over a single centered driver, you can hear the difference in terms of image stability with respect to head movement.

As for the ubiquity of MTM centers, that is a model supported by its visual, rather than its sonic, impressions for accomodation to those with the common space constraints. The only good centers have vertically stacked tweeters and mid-ranges. A standard speaker configuration is even better.

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.