Some questions were raised on Qts of a driver and why the Qts of the IB15A and SBP15 is only around .4 and not in the range of .7 or more. There are two answers to this question. The first is that the Qts of the driver is not the critical factor, the overall Qtc of the system is. When any driver is put into a box, trunk, infinite baffle, etc, the final Q of the system is what is important. In the case of a single SBP15 in a 15 cubic foot trunk, the final Qtc is about .5 or a pair gives a qtc at about .59. As you put things in the trunk, apparent volume is taken away. If you have a pair of IB15's and your trunk is half full, the Qtc goes up to around .75. This changes the curve and it starts to roll off sooner and sharper. If you start with a driver that has Qts in the .7 range, as you decrease volume this Qts can skyrocket quickly to the range above 1.0. This greatly rolls off the low end and can give a very boomy sound as the driver is not at all well damped.
Take a look at the following. This shows the SPB15 with Qtc of .58 (pair in 15cf trunk) in white, Qtc .75 in green with reduced area, and theoretically what it would look like in that same reduced area if it had started with a Qtc .7 and gone up red curve.
Others have even smaller trunks and want to do a single SPB15 infinite baffle. This lower Q of .4 allows for that. It also means the driver has more motor strength and efficiency. You start out with 90.6dB 1W in the upper bass region where there is little cabin gain. This is much more efficient that most drivers, and this is the area where you really need it.
The second answer is that it is simply better to get the most output with the least amount of power in. At 20hz, you only need about 250W to reach Xmax with the Qtc of .5. With a Qtc of .7 you need about 600W input to get the same level. This is more than double the extra power to get the same output. This leads to more thermal issues and more current means more flux modulation, so distortion is higher to get the same amount of output. Any time you can get more output with the same input power, that is a good thing. It is much easier to EQ down the low end if you have too much than it is to have to put in more than 2x the power to get the same output.