Hi Tom,
Don't know if you have any comment or not, but I find the performance of the XV-15 very interesting in the below tuning/infrasonic region. Obviously below tuning, a vented sub is fighting a losing battle with physics; most manufacturers tend to just filter out content below tuning as a result.
However, the XV-15 seems to do better than average in this regard to my eyes. Now obviously it didn't pass the 10Hz CEA 2010 output test (nor is it reasonable to expect such a thing). On the other hand, the XV-15 still is pushing hard into 10Hz in the power compression sweep with maximum output of 82.3dB. Compare that with the sealed Epik Empire sporting dual 15" drivers which puts out 80dB in the same test, and I have to admit to being impressed, regardless of the distortion produced; it must be a heck of a driver to tolerate that sort of load and still walk away. I'd also expect the more gradual roll off is what pays dividends in regards to group delay and as a result, the waterfall.