If you want to save some cash do not go with JL, there are a lot better options for the buck, especially if you are looking to add a little bump.
Rockford Fosgate would just be one off the top of my head.
I second what iron said, by using an AUX input your loosing a lot of quality including bass. Even upgrading your head unit will help with audio quality. Also will be easier to install your new sub and amp.
Sub
http://www.crutchfield.com/p_575P1S8...-8.html?tp=111
Amp (could go bigger)
http://www.crutchfield.com/p_575P400....html?tp=35834
Also if you buy from amazon or woofersetc they would even be cheaper.