Speaker on its side as a center?

I am curerntly using a B&W 804 as a center with 2x 804's as mains. 2x 805's as rear. The center 804 was one of a pair, but I lost one courtesy of the shipper. I would like to lay the center 804 on its side under the plasma TV. Idea is to have this on some kind of a shelf structure under the TV platform. Any thoughts on performance of a speaker on its side? thanks.
Not a good idea. You'll be relying on the speaker's "vertical" dispersion, which is usually much worse than its horizontal dispersion. Of course, most people use horizontal center speakers, which suffer from the same problem. The best center speaker is another of the front speakers, mounted vertically just like the others.
In my view, all center speakers are compromized by their position above or below the TV. However, most of the material played by the center is dialogue with some special effects. The best position is to get it as close as possible to the height of the mains with the tweeters at the same level. Being on its side is less important than any other factor. Good luck!
I use 6 identical speakers for my 6.1 system. I was laying the center on its side for esthetic purposes, but tried standing it up to hear the difference. There definitely was one. Tweeter dispersion patterns and all, I believe.

Only way to know is to try it out and see what works for you, your room and your speakers. I would certainly give it time in any position to see what you think. It's not really an A-B in 5 minutes type of test IMHO.

Try it. It will compromise horizontal radiation but only you can say if it is acceptable. I wouldn't do it.

its geerally for dialogue.....no big deal
Well if I actually had one and was in your situation I would have already tried it.---I wouldn't need anybody's permission.---I mean,how bad can it be?? Stick it on something like a coffee table or a box the size you need and give a try. Then decide on a stand. 3 identical speakers across the front can't be all that bad.---Just keep an eye out for the center channel police.
You might check with B&W to see if there is any concern from their end. If the speakers have any ferrofluid cooling that may prompt you to rotate the drivers. I had a freind that did it with his old Kappa &'s and the Emit tweeter failed about 3 mos. later. Upon reading the manual Infinity instructed the user to rotate the tweeter when laying the cabinet on it's side. Just a thought.
"Jaybo: its geerally for dialogue.....no big deal"

Really? Try some of those 3channel Living Stereo or Living Presence SACDs and see if you are still singing this tune.

Cobbled an old Celestion as a centre speaker. The other one of the pair has a dead tweeter that can't be replaced, so I think giving this poor lonely speaker a second chance is a great gesture. A mono system would be the other use for it, but I ruled that out. Sounds OK to me, heck it's only for TV!