One of my favorite torture tests is the WOTW Pods emerge scene. Here is a comparison of a single MFW-15, Dual MFW-15's and a single PB-13 Ultra.
Note what the Ultra is doing in the 13-17 Hz range. For a single, 13.5 inch driver, this guy is incredible.
Also ... what makes this scene so interesting is that rather than a simple sine wave, the sub is being asked to deliver multiple frequencies simultaneously.
This was done using the Peak Hold function, and gives an idea of max SPL across a wide frequency range.
This was taken at the listening position - about 13 feet away, in our upstairs theater room. Both subs are put in the same position, and of course, both will realize the same room gain from that position.
The Ultra:
Single MFW-15:
Dual MFW-15's: