Increase the amount of bass that is going to the ported sub box by increasing the bass frequency decibels. All head units will have a bass control that can be increased, but many high-tech car stereo systems include a graphic or parametric equalizer that will allow you to boost certain frequencies. Boost frequencies below the crossover point, in the 60 to 100 Hz range, on the head unit or equalizer for the sub box.
Adjust the crossover point on the amplifier for the sub box. Most amplifiers allow you to use a built-in crossover when the amplifier is bridged for sub boxes. Raising the crossover point a bit will give you more bass. If the crossover point is set too low, the bass will be hard to hear and the sound waves will need further distance to be heard. Raising it to about 100 Hz will increase the bass that is heard in the vehicle.
Replace the speaker with a more efficient or higher-quality model. Upgrading the speaker will add some bass, but it will also increase the quality of the sound in the low frequencies, which is desired. Change the speaker using the dimensions of the box as your guide if the sub box and your speaker are matched. The box is designed for that particular speaker and the new speaker must match as well for efficiency of air movement.
Fill the sub box with polyester fiber fill. This material is used to make the speaker function as though the sub box is bigger than it is. Remove the speaker and spray adhesive on the walls of the sub box before adding the polyester fiber fill. Press it down so that it sticks to the inside surfaces of the box.
Change the speaker if it is not designed for your sub box. The amount of open air space in a sub box is determined by the speaker specifications. If you use the wrong speaker with the enclosure, the bass response may not be what it could be. Use the dimensions of the box to determine the correct speaker size and specifications for maximum bass response.