Mike, if I'm following, you say that you turn the level control on the sub "down to half way" when this happens? If you mean that usually it's set near max, that's too high. The setting after calibration generally should be half or less and the sub out setting on the receiver should be higher to compensate. It's possible that the sub amp is defective, but try changing the relative level settings as described.

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Enjoy the music, not the equipment.