Geoff, it's a widely varying voltage(not current)from the sub pre-out of the receiver that makes the difference in loudness. The output into the internal sub amplifier might vary from a full volt or a bit more down to a small fraction of a volt for more quiet moments. The sub amplifier then takes this voltage and increases it about 30 times(a "gain" of about 29dB)so that it's high enough to operate the subwoofer driver.

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Enjoy the music, not the equipment.