Make sure the volume for the subwoofer is turned up.
On your "home" screen, select "controls."
On the "controls" screen, select "system trims."
On the "system trims" screen, select "level trims."
Here, you can choose the subwoofer and adjust its level (upwards). Hopefully, it will be able to increase sufficiently to satisfy your requirements.
[add] To get an even greater effective increase, try turning down all the other levels -- that should make the subwoofer sound even more prominent.
