I'm actually quite impressed by the amount of bass from the NAV/JBL sound system. I find myself turning the bass down a couple of notches from center because it feels too overpowering sometimes.
I also wouldn't want to put more unnecessary things in the already small hybrid trunk
I've had 6", 8", and 10" Bazooka tubes in previous cars and have been impressed by how much bass they put out - if you're looking for a different thing to try, I'd recommend you try a non-powered 6" hooked up to an external amplifier. The built-in amps seem to distort too early. They work best pointed toward the back of the car, not toward the sides.
