Depends on what kind of improvement you're looking for. You can start by wrapping and stiffening the sub enclosure then changing out the speakers.I am looking for any suggestions on upgrading the stereo system in my 2013 grand cherokee alpine edition it has the harmon kardon 19 system in it anybody out there system yet
If you are game, you should check out the JBL MS-8 Audio Processor. All you have to do it connect your fronts and sub into it and output to the existing speakers (or external amp then speakers) and let it do it's thing. I'm not sure how much better it will be than the H/K system as both are made by Harman (Harman owns JBL) and it MAY already have the base tech in it.