Nope, that's external dimensions.
I got the idea from this
http://www.data-bass.com/data?page=system&id=91
He's using a 15.5'' cube box for a 15'' driver, and the heavy duty drivers he tests produce quite excellent maximum output. I just wanna do something like that, but with a little bit more enclosure rigidity, so I'll make my box 16'' cube. Plus, I think a "bezel-less" sub looks pretty cool!
I don't push my subs hard. The Zv4 looks like an extreme duty subwoofer, and I don't think it'd be fazed by the 2000 watts from an iNuke amp if it somehow ever needs it. I'll take the smaller size over efficiency anyday.
You do realize a 16" cube box is 1.76 cubic feet gross internally right?
If you're going sealed, you could add a ton of stuffing and you might be ok. But it's not gonna be great. Also note that Sundown doesn't recommend sealed for the Zv.4's. And forget about ported in that box.
I do realize that. I'm just trying to build the most powerful small subwoofer I can. The Zv4 actually models really well for small sealed boxes, even better than the LMS 5400. At one point I considered doing a LMS-R 12'' with dual 12'' PRs in a 16'' cube with a 20Hz tuning, but then the LMS-R 12 is no longer available, and people suggested the 15'' Zv4 sealed instead as I have a large budget (not a lot of love for PR designs on the forums for some reason)