Have you thought about 6 1/2 subs? Kicker makes a nice one that I've heard. A friend has a pair of them in a ported box in his Miata with a 400 Watt Kicker amp and they sound AWSOME! I would like to put them in my car. I think they will fit with a little effort.

BTW, I'm running a high quality Aftermarket Head Unit with the factory speaker and it sound fantastic. I think the front speakers are good enough. The subs actually sound really good with the aftermarket head unit, they jut need a little more juice. I think a good amp on them might work as well.

The Amp setup I had in mind was using a 4-Channel amp. The reason is the the dual voice coil subs. You can run each sub off one channel by connecting both voice coils to one channel. This of course creates a 2 ohm load on the amp with draws some more power out of it. So, you would be running each door off one channel and each sub off one channel in 2 ohms. It will work fine and may sound really good. So, if the factory subs don't do what you want, you haven't lost anything and you can jjust add an aftermarket sube later.

I hopoe that all made sense. I ramble sometimes.