The EX flag wouldn't affect performance of the center channel - it just messes with the surrounds and the surround back (rear center). What it could be, though, is a driver in the center channel (probably the tweeter, but possibly a mid/woofer) that has either failed or had some significant wear that is degrading its performance. That behavior would appear any time the center is used - both on DVD's with 5.1 tracks that have significant content in the center channel and on stereo sources processed with Pro Logic which often steers a lot of material to the center. The weird stuff from the surrounds may be unrelated and could even simply be a result of an unfamiliar surround mode (NATURAL).