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Oh, and, in particular, I have no trouble whatsoever understanding TV shows that were recorded more than ten or fifteen years ago

Because those aren't mixed in surround sound and thus don't have a center channel.

Another possibility is a phase problem between the left and right speakers in a regular stereo mix. Like if the wires for the left speaker were reversed, you'd have the left speaker out of phase with the right and anything that's equal between the two channels (like center dialogue) would get phase-cancelled, while stereo-panned sound effects would be rather loud.
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Tony Fabris