This was asked in another forum, and as I've been playing around with my center channel position for awhile, I thought I'd repost my experiences:
I originally had my center speaker atop my television, and I noticed an improvement when I changed my center placement. However, I want to stress that it wasn't simply moved from above to below the TV, but rather from above to below
and in front of the TV, and on a
real speaker stand.
It's not hard to understand why the improvement in my circumstance. A television set makes for what must be among the worst speaker stands in the world, essentially a big plastic box filled with unwanted resonances. Having a speaker sitting on top of such a thing is by no means optimal, to say the least.
So I purchased a stand, a Wood Technology 12" one, that put the speaker just below the level of the screen and which also angled it up to the listening position. This resulted in a noticeable improvement in sound, but was still not quite ideal. I found the 12" stand, though an improvement over the TV, to be too close to the floor. Although floor reflections didn't seem problematic, the center tweeter was no more in line with my main speaker tweeters than before - just beneath them by about the same distance that it was formerly above them. So I got a new stand for the TV to raise the screen a bit higher (it was also a tad too low), and switched to an 18" wood tech stand for the center.
Now, this was really an improvement! While I still didn't achieve a perfect tweeter-to-tweeter height, it was much closer than in either of my previous two attempts and all the unwanted resonances of having used the TV as a stand where, of course, gone.
For me, it was less an issue of above/below the screen as it was finally putting my speaker on a real speaker stand and finding the best compromise for aligning the tweeters.
Though I'm not necessarily recommending this retailer (I don't know a thing about them), you can see a good picture of the stand(s) I'm using here:
http://www.uniquestylesolutions.com/wmt-ct-series.asp They're not as sturdy, nor do they boast the mass of the Atacama stands I use for my rear speakers, but they were the only center speaker stand I could find at the time (this was about a year ago).