i already tried exchanging the standard subs with the Bazooka 6.5" Dual voice coil subwoofers from their bass tubes.

the standard subs have very little depth, i would guess about 2".
and you can't install a woofer with more depth as the subs are installed at an angle inside a plastic frame.
but i tried hooking up the standard subs to an amp and they can withstand more power than the factory amp.
so i guess that is one way to boost their output a little.