The PB12 Plus/2? http://web.archive.o...b-box-plus2.cfm
That doubles the chances for vibration. The driver is the TC Sounds db500 (db for Deon Bearden, the designer) http://www.caraudio....db500-12-a.html Moving mass of each driver is 102.7859g. Even though xmax isn't listed here, it's right around 20mm each way. Let's say you have a 30hz tone that pushes the driver to the excurson limits. That means there are 200g of weight moving 40mm, stopping, reversing motion, moving 40mm, stopping, reversion motion, moving 40mm, wash, rinse repeat. 30 times every second. That is a lot of kinetic energy every second.
Since yours is downfiring, that will lift the cabinet ever so slightly each time. Your subdude cushions the blow each time the cabinet weight is put back down on the floor.
Those are great subs. I built a pair of passive cylinder subs using the SVS Plus drivers that I picked up on the used market. I traded them to a guy for a custom built TC Sounds based 18" driver.