You will want to read the manual for it, but rojo has given you the essentials. One port plugged = deeper bass. Two ports open = louder deep bass, but not quite as deep. Like Rojo said, try it both ways and see what you prefer. The tow ports open mode is really only advantageous is you listen at very loud sound levels. One thing to keep in mind, there is a switch in the amp plate that you will want to keep aligned with whichever mode you are running in. If the ports are both open, make sure the switch is set to "25 Hz", but if one port is plugged, make sure it is set to "20 Hz". One thing you do not want to do is have both ports open with the switch set to "20 Hz", that endangers the driver in loud deep bass sound. On the other hand, you can have it set to "25 Hz" with a port plugged, but all that will do is lessen deep bass output, however it is safe. If you are interested in a closer look at the performance difference between the two mode, there is a very good test comparison
here.