Small Sub for 2010 Prius IV with JBL and Nav?

Hello, I've been thinking of upgrading my sound quality a little bit. I was thinking of addin g a small (slim) sub woofer either underneath the driver's seat, or in the storage compartment above the spare tire. Don't really need a big sub that would take any of my trunk space, and also I have no intentions of waking up the whole neighborhood with massive bass. So as usual, I have a couple of questions
In the Gen 2 Prius the navigation DVD was underneath the driver's seat, so there was no room to install a small sub there. Since the map DVD is in the head unit now, I would assume there's no DVD Rom under the driver's seat in the Gen3. But I looked and there is something else underneath there. I didn't investigate much, but what is that? Is it possible to plug a sub like one of those

into the stock JBL system?

 

Hmm. I thought the amp was under the passenger seat.