I finally got around to installing a subwoofer. I went with an under seat JBL BassPro Nano. It puts out a pretty good amount of bass, considering it's compact and fits under the passenger seat perfectly. I have a manual seat and I can still lower it all the way with no interference. It fills in the bottom end quite nicely, probably how it should've been from the factory. It's not going to blow away the car next to you, but it's sufficient for full sound in the car. I gave up on the hatch subwoofer due to lack of space. There's only 3" +- depth, and due to the non flat surface, there's no way to mount a spacer ring. I could've gutted the factory sub and used the bracket, but I didn't have the heart to do it (yet). Someone with more skills could probably make it work with 3d printing a mounting ring.
The sub is available here:
JBL BassPro Nano
I didn't take too many pics, but the install is pretty straight forward. I ran the power wire under carpeting, and under and around the big battery cover to the 12v battery in the rear. I grounded to the chassis under the door sill, and tapped into the rear speakers Metra adaptors and ran the wiring through the factory b-pillar harness. There is an opening on the bottom of the harness that is perfect for this. You do need to take off the rubber grommets and unplug the connecter to do this. All the wiring (save for behind the rear seats) is under the door sills and ran under the passenger seat. There is a hole in the carpet you can fish wires through. I tried tapping in behind the head unit first. You get sound, but there is no volume control, so don't waste your time. I believe I read this on another thread too. The sub turns on with speaker level inputs, so I didn't need to run a remote-turn on lead, which was great. Because I didn't want to tap/cut into any factory wiring.
Overall, a worthy upgrade. A pro (and maybe me) could've done a little cleaner install. But that would've required stripping most of the interior to hide the wiring better. Something I really didn't feel like doing. I'd say B on the sound quality overall, and my hearing isn't the best. I'd say the limiting factor is the factory amp/ equalizer. You'd basically have to bypass it, but looking at the wiring diagrams, a whole lot of things run through it, so probably not possible. It works for me, for now.