6 or 7 o clock is barely on why have such a powerful sub if your not going to use it? Or when does one ever turn it up?
Like EFalardeau said the reason for the gain knob on the subwoofer is to balance it with the rest of your system. Different receivers and preamps can have different output strength going to the subwoofer. The gain knob allows you to adjust the signal going to the subwoofer so that the subwoofers output is roughly in balance with the speakers. If you had a very weak receiver you would turn it up more but since you have a good receiver you don’t need to.
From there you fine tune all the speakers and subwoofer with test tones and an SPL meter to balance them all. And then if you’re like me you write that down to use as a baseline (so you don’t have to recalibrate again) and then start turning up the subwoofer and surround/back speakers higher where you really like them.
The filter will cut out whatever is below the level it’s set for 20Hz 15Hz not sure. I guaranty you that sound in the 20-25-30-35-40 Hz range will shake the crap out of your room and no sane subwoofer manufacture is going to filter those frequencies out.
