In my experience, matching the center to the mains makes a BIG difference. For a while I was using a center (NHT Audiocenter 2) of higher quality than the mains (Advent Heritage). The timbre difference during pans was obvious and significantly detracted from the spatial illusion.

I agree with others in this forum who mention that the rears don't seem to make nearly as much difference - most movies don't use the rears much and virtually never for voices.

Many movies and music videos on DVD seem to put most of the acoustic energy into the center channel. While bass management can help, in my experience musical material on DVD is best served by the best center channel you can afford, with the best LF extension. I went one step beyond and got a *pair* of "bookshelf" speakers (Celestion A1) that match my mains (Celestion A3) and use both, wired in series, as a center. I have yet to see a dedicated center that can match this sound, and the pair of bookshelf speakers is much less likely to go obsolete.