A few months back in the ULF thread,
posted the results of a very, very interesting experiments. Basically he measured how much his couch was actually shaking using a smartphone app designed for earthquakes.
He noticed that despite being smaller and EQ'd flat as were his front subs, his back subs were making the couch shake more with the same input (15 Hz sine wave I believe). He proceeded to rule out many likely explanations. The output was the same as measured in dB SPL -- same FR from both sub, same level, measured on the couch. The couch was even moved at different positions in the room. The signal was a single tone under the modal regime of the room.
This lead us suspect that SPL measurements did not tell the whole story, and it brought us into very interesting topics of acoustics. One of the likely cause of the strange results was that the back subs were ported while the front subs were sealed. At low frequencies, almost anything in a room can be considered "near field", and thus conventional acoustics (which mainly deal with traveling plane waves) break down. When plane waves travel, one can infer the acoustic power, acoustic intensity, etc. from SPL measurements alone. But sound waves have two linked quantities: pressure and particle velocity. The relationship between the two can be completely different when near field where sound waves are not planar.
Long story short, it is suspected that close to a ported sub at a frequency near tune, the PVL (particle velocity level) is probably much higher than what is assumed from the SPL measurement, leading to a greater acoustic intensity -- which could make a couch shake more.
Another way to look at it is that near their tune, it is likely that ported subs are grossly out of calibration when considering sound intensity instead of only SPL.
The problem is that sound intensity sensors are special devices that are not readily available so nobody could confirm the suspicion we had. Perhaps PSA could buy a sound intensity probe and characterize ported vs sealed for us