Independent Subwoofer Adjustment?

First off I would like to say I love the JBL audio system and overall have been very impressed with how good a factory system can perform. I'd like to replace the door speakers at some point but until then, I'd like to be able to independently adjust the sub. I'm not talking about turning down the "bass" in audio settings, because then I lose the bass in my speakers.

Can I keep everything stock but add something on (EQ, X-Over) that will allow me to make the adjustments I'm looking for, or will I have to buy a new head unit to do this?

Any help would be appreciated.

 

I'm pretty sure you would have to replace the JBL stock sub amp with an aftermarket one to have a bass knob.
Edit: I thought the JBL had a separate amp for the sub but it appears there using a 8 channel all in one so you would need to do as I mentioned in post #6

 

Bump, I have JBL package and I want a little more tune-ability. I was also looking to replacing the tweeters and sub would this affect the quality of sound?

 

Yep aftermarket sub and bass knob is 5he only option