Larry, Doug(bug)is right on in his reply. The "subwoofer mode" setting on your 510 does the same thing which most HT receivers do with a "small" setting, i.e. it rolls the lowest bass off from certain speakers and instead directs it to the sub. For example, using Mode 1(which you probably should)takes the bass below the crossover which you set(typically 80Hz)from all the speakers and sends it to the sub. This is the same thing which results from setting all the speakers small.

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Enjoy the music, not the equipment.