First things first. Treat the room first then say there is not enough bass. You guys shoot first and ask question later. Spend your money on a little education FIRST that will cost you a little time but NO MONEY.. Then you won't be spending money good after bad.
Learn how to use some great inexpensive tools to learn what you need to do first.
I assure YOU it will pay great dividend in the end..
I owned a few JBLs. Rythmik and GR are my ONLY recommendations with those speakers. All of the bottom firing sub are worthless unless they are slot loaded and decoupled.. JBL makes a great speaker, don't screw it up.. Caps, resistors, inductors and coils, is always a good idea.
Get a book, take some measurements, Treat the Room. HiFi 101.
Most of all enjoy some of the best speaker ever made..:-)