If all you want is the speaker without the built-in amplification, M&K make a 5-channel variant that would sit under your plasma.
Alternatively, have a look at the products from Artison. I know them and think they're pretty good, but they don't make underslung speakers, only side fitting ones.
Have already looked at the Yahama but want to use my amp that i already have and only want front and centre speakers.I believe these incorporate an amp and rear speakers.