Man, at this point, I'd just buy a different brand sub to put in the box.. the box is really where all the $$/time is anyways.
I've been thinking about that pretty seriously. It looks like the JL Audio W0 and W1 drivers would fit. W3 and up probably won't.
TBH, if the enclosure made me feel like my money was well spent, I would have just shrugged and tossed a better driver into it quite some time ago. Time is money after all.
I have yet to install my NVX boost for Model 3, so hopefully it's not something actually about NVX. Mine came in a clean box, no damage, looks brand new. When I eventually get it installed, I'll report back.
However for your scenario- something is off there. First one could be bad luck. Second one is a refurb/shipping error and already bad. Third one? Something seems really off. If you were certain that it was on low volume to start, then I'd double and triple check the install and get out some tools to measure waveforms.
The NVX subwoofer itself is the same they sell to thousands of people, and I have no reason to doubt it's performance. It can handle 500W without any problems.
One thing worth noting that is not obvious- the box comes wired for 2 Ohms from the factory. It's a dual coil design, and they are wired in series by default. If you've got a massive amp behind it, you might be blasting more watts than you expected.
I'd recommend making sure your subwoofer is good NOW. If enough time passes before you find that it's bad, they will make it difficult for you to get a new one. I waited to do my install and managed to get my first return approved on the last day that I could have returned it. They told me if I had waited one more day before I called them, I would have been SOL. (and I had an email in to them. they were ignoring it, so I made the phone call)
I agree that there's something terribly wrong here. That's why I've spent hours of my time testing and troubleshooting to be sure I know what's going on.
The sub is wired for 2 ohms from the factory, yes. The coils are wired in parallel, not in series, they're 4ohms each. My amp is 500 watts with a 2 ohm load, and is (or was, Alpine doesn't make this one anymore) an option in the kit when ordering from SonicElectronix. I used a DMM to confirm that I wasn't sending >500 watts to the driver when I did the install the first time. (I used to HATE it when old men said things like, "I've been doing this for 30 years!" but here I am, having been doing this for 30 years... lol) The first subwoofer driver they sent me is my first blown speaker in any of my installs in all that time, so I triple checked everything about my install before I even reached out to them for warranty coverage, because blown voice coils generally are NOT something that you can get covered under warranty.
I honestly don't know what to think. It certainly seems clear to me that the quality of NVX subwoofers is suspect, but I've only dealt with 2 of them that were new, (First and third) The second one was clearly someone else's RMA'd subwoofer that got shipped to me rather than a new one, and that one had the exact same problem that I'm having with this third one. (I can also say for sure that if I let this third one play for 2 hours, it will be burned out like the first one was. I could already smell it burning when I took it out of the enclosure to see what was going on, and that was after 5 mins of play time at 25% volume)
If you push down on the speaker cone, you can feel the voice coils dragging on something inside. That's what's wrong. What I don't know is whether that can likely be caused by shipping damage that destroys the cardboard box but doesn't show any visible damage to the fiberglass or carpet on the enclosure. I seriously doubt that it would happen with any of the higher end, more expensive drivers I've used in the past.