I'm hoping to upgrade my current front and possibly center channel speakers, but I need to spend under $400. My front speakers are a pair of Polk TSi 100s, and my center speaker is a Polk CSR, and my amplifier is a Denon AVR-S720W. I don't have major problems with the speakers, except that the sound can sometimes be a bit muddy, especially on dialog. Any upgrade suggestions that won't run me over $400 would be greatly appreciated.