BW, the purpose of that screw is to connect the chassis of the sub amplifier to the shield of the sub cable. Ordinarily this might allow for a slight further reduction in overall noise level. However, if a ground loop is present, the resulting hum fed into the sub cable overwhelms any other factor. Removing the screw still leaves the sub amp grounded through the three prong wall plug, but cuts the loop which could exist through the sub cable.

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Enjoy the music, not the equipment.