In my 2013 Toyota Prius Plug-In, the JBL GreenEdge system is okay… I needed some additional bass, however, I’m not trying to win any sound competitions.
Recently I upgraded the front speakers in my Prius to some nice JBL Speakers. The speakers didn’t give me what I wanted… A little more bass!

After some measuring and searching online I found a nice subwoofer that fits perfectly under the passenger seat. I mounted the OEM JBL Amp to the bottom of the passenger seat then set the subwoofer in its place.
I choose the Rockville SS8P 400w 8″ Slim Under-Seat Active Powered Car/Truck Subwoofer Subwoofer. For this installation, I also needed a LOC Line Out Converter, an RCA Plug, and a Male Plug Cigarette Lighter Adapter.