06 530xi, I need more bass!

I find the bass to be lacking in my 530xi, after reading a few informative threads it looks like the Earthquake SWS-8 is a worthy replacement for the under the seat subs (not sure if I need 2ohm or 4ohm though), and what replacement amplifier(s) work? Where do you go for them? Are they plug'n play or a little bit more involved? I would like to avoid throwing a code. I was also contemplating just putting an external amp in the trunk for the two 8" subs. Any thoughts are greatly appreciated.

Here is an option for you, it's what I'm running and it has more bass than I really need. I have a JL 12" W7 sub with a 500 watt amp in a custom drivers side trunk enclosure. Below is a link to what it looks like. No codes thrown and bass for days!!!! Hope that helped. http://www2.crutchfield.com.edgesuit...3694281-o.jpeg