okay, so your head unit has crossovers!
It shows how to set the crossovers on page 10-11 of the "operating instructions" manual-
http://esupport.sony.com/US/p/model-...ls#/manualsTab
I'd set the HPF (high pass filter) to 100hz.
A high pass filter electronically filters the bass out of the information being sent to the speakers. This is way better than just turning the bass down to make your mids/highs perform better.
Using a HPF completely eliminates the bass, so you can still use the radio controls to turn the bass up and down, and not worry about distorting your front speakers.
Earlier I was telling you to "turn the bass down". That's a rude/amateur way to make your fronts sound better. Using an HPF is the proper way to keep the bass away from the fronts.
To install a powered sub you will need a good size power cable going from your battery to the sub. You also need an RCA cable and a remote turn on lead running from the back of your deck to the sub. You also need to find some good solid unpainted metal to use as a ground for the sub(somewhere in the cargo area)
For someone who's done it, it's no big deal. If you've never done it, it can be intimidating. But it's easy...
Figure out how much time/work/money you want to invest in better tunes.
Adding a sub is certainly a lot more work than just replacing door speakers, but the sound will be way better.
I tend to kind of ramble, but I'll help you figure this out! Keep shooting me questions and I'll keep answering. If you have specific questions that need a specific answer number them for me, it helps to keep me from rambling on...