I think its best to set your sub at 0db on your prepro and adjust the volume knob on the back of the sub to 75db.

This makes it easier to pump up your sub +5 if you want, or turn it down -5 with the prepro, and you know where you are, because "0" is 75db.

This is just what I did, right now I run my sub +5db, so all my speakers are calibrated for 75db, my sub runs in at around 80db.

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