Me being an fanatic -
The stock speakers are pretty decent, I've got just the sport package on mines without the JBL upgrade..I'm powering them with a Pioneer-3200BT Headunit and they are pretty damn loud. I would recommend going for a pre-amp output like Supercharged Says and get a decent woofer..
Make sure when you get the woofer consider this:
Quality Bass = Sealed Box
Hard Knocking Bass = Ported
And do NOT buy a pre-made box from online whether it's $20, or $50, spend $100 and the box will be the biggest difference you can hear and feel. My boy just made a custom box for an 8" woofer, and his almost hits as hard as my 12" Image Dynamics.
Also - if you are getting a woofer consider the Ohms and the amps
If you get a monoblock woofer, make sure it is Ohm Appropriate..
I.E. 4 Ohms can be wired down to 2 Ohms for one woofer
I.E. 2Ohms can be wired to 1Ohms
If you need help wiring, go to
www.12volt.com