I brought and installed this $129 8inch active subwoofer
Under Seat Active 8" Subwoofer - Jaycar Electronics

I didn't expect much and was going to take it back if it was hopeless, but I've been pleasantly surprised by how well it works.
The factory fitted speakers rattle and one sounds like it got a rip in it. So my plan was to turn down the bass and let the subby do the bottom end work which I now done and it sounds a lot better!

The brown notes aren't quite as good as hoped and I suspect an 8 inch driver isn't ever going to cut the mustard at this frequency, but over all the sound has fattened right up.
I can recommend this under the seat option for those don't want to use up boot space and just want a lift on the audio front.

Btw, I did the rear aux and line out mod by populating the pins in the carrier on the side of the Head unit. While the rear aux worked, the line out didn't (anyone know why?).