Bryan, the concern that you describe shouldn't be a major one. If you want the sub trim on the receiver to be significantly higher than -10(although it's possible that the 990's sub output is higher than most)so that you're confident that enough voltage is being fed in to keep the sub amp "awake", turn the volume control on the sub even lower than 7:00. There's no disadvantage to this, as it just cuts the part of the incoming voltage that actually winds up being amplified in the sub amplifier down farther.

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Enjoy the music, not the equipment.