John, maybe the "custom configuration" that you refer to has already tried this, but note that the "formant" frequencies of speech, from roughly 500-3000Hz, contribute most strongly to intelligibility. So, if you've not already done so, besides bumping up the center channel maybe 2-3dB over what the auto calibration set, also boost those mid frequencies in the center channel maybe 3-4dB.

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Enjoy the music, not the equipment.