"Qts - This is the total strength of the driver resonance. It is a combination of electrical Q (Qes) and mechanical Q (Qms). A speaker with high Qts will have a very stiff suspension."
As I understand it, speakers with stiffer suspensions are better suited to infinite baffle (free-air) applications as they are less prone to over-excursion.
The actual bass output is derived from the conal surface area and Xmax (excursion capabilities) of the driver. The more air the driver moves, the more bass you'll create.
This sub has a pretty high QTS of .8 which should help keep it from being overdriven. And, it's Xmax is also pretty darn long for so small a sub at 8mm (not a point-to-point measure which some manufacturers use to exaggerate specs).
BTW, I believe this driver was designed to be a free-air 6x9 sub.
I suspect 4 of them could move quite a bit of air with even a very modest amp (something that'll produce 100x2 @2ohm stereo - a dime a dozen amp). And, zero loss of trunk space!!!
