If you have the sub connected from the subwoofer pre-out on the a/v receiver or a/v preamp to the LFE in/line-level in on the subwoofer then you should be able to adjust the volume(gain) from both the subwoofer volume(gain) dial and the subwoofer gain setting on the receiver or preamp which shows up as number ranging from -12 - (+12) which represents the voltage(gain) signal being sent to the sub. The higher the number the more voltage is being sent to the sub.
When dialing in a sub with a SPL metre you are trying to strive for a happy medium between these two settings. Half or less on the dial(gain) of the subwoofer and a -3 to (+ 3) setting on the receiver/preamp for your calibrated setting.
