Not all shows on TV are Dolby Digital.

What DSP mode are you using for the shows that are not DD? What is the treble/tone controls setting on the TV?

I don't think the X-CS will match as well. With any subwoofer, it does matter what the fronts are since the subwoofer has nothing to do with the midrange and treble. I would call Ascend about the use of the 340SE center in your situation.