I tried mine on the tv stand in front of the TV (it blocked the remote for tv operation) and on the shelf under the TV and it sounds far better up top.

I can understand why you would want your front or main tweeters at the same level. I have my rears at 40" and that is also much better sounding to me than they were at 30".

I also notice in the gallery than many others have their center speaker on top of the TV. And they obviously make shelfs and stand just for this. Not sure about problems with plastic but mine is aluminum so it's not a problem. The speaker is just heavy enough to keep it steady and it would take quite a bump to cause it to fall. I can slide the whole system away from the wall and it stays put just fine. And did I mention is sounds better?