To each his own, but I just don't find even the stock sub worth it in this car. Don't get me wrong, I love music, and I like to listen to some while I drive. But if I want a top-end audio experience, I've got that at home
I pulled my stock sub out and replaced it with nothing, and turned up the bass a little bit on the audio settings of the head unit. Sounds fine for driving music, even if it lacks a little bit of bass punch. It's not like a car with as much road noise as the Z is ever going to be a rolling concert stage anyways (at least not without adding a lot of weight and going against the nature of the car, imho).