Here is a close-up photo of the Z-base 575's port for the onboard subwoofers. This augments the bass extension of the two down firing 6.5 inch subwoofer. A long tube is used to direct the sound from the subwoofers to the external rear port. This design provides for more powerful bass at low frequencies. In other words, you get the effect of a larger subwoofer without having a larger subwoofer.
On the other hand, if the user desires, an even more powerful external subwoofer can be connected to the Z-base 575 using the audio output connection shown in the previous photo.

Proceed to the next photo...