IMHO, I would never place anything on top of a speaker.
You didn't specify the height... Since you are getting the table stand, I'd recommend placing the center channel speaker right in front of the stand with the back of the speaker flat against the stand. There is enough space between where the plasma screen sits and the stand (1cm) to run the speaker cable through.
You didn't specify the height... Since you are getting the table stand, I'd recommend placing the center channel speaker right in front of the stand with the back of the speaker flat against the stand. There is enough space between where the plasma screen sits and the stand (1cm) to run the speaker cable through.