Factory subwoofer?

I recently bought a 2009 Tacoma trd access cab, it seems to have a factory subwoofer mounted in the back. It has a factory six disc cd...the sound of the subwoofer is horrible, does anyone know if this is a factory option and if so what brand name is it and is there any way to get a good sound out of it? I have a 10" alpine to go in and a custom ported box and a new amp but just curious about that subwoofer....

Also if anyone has any suggestions on a deck..something double din? Really not sure what to do with this one, so disappointed with the sound of the subwoofer that's in it.