From what I've heard (only have personal experience with the X and Zv4, never a SA or Zv3), the Zv3 is still a very capable sub. The web page states that the X and Zv4 has a different kind of spider that reduces distortion at higher frequency, and I definitely notice and like the sound. Some people prefer the more traditional sub sound and they would be better off with the Zv3 as it is more "normal" sounding. You said in the OP you're after a lot of low-end output and I think the X is better suited to that. If you were building a box tuned to maybe 45hz or something like that I would think the Zv3 is better. But like I said, I have no personal experience with that sub, just a lot of reading and used the X and Zv4, so take my interpretation with that in mind.