You can follow the steps given below to get an idea,  1. Start out by playing a reference song or any other song you know well.  2. Your subwoofer will most probably come with a manual or guide. Find out your main speaker’s low frequency cut off and set your subwoofer point 10 Hz higher.  3. Play the track again. Adjust the volume of the subwoofer until it reaches a level you are comfortable with.