Second the motion on switching speaker cables and seeing if the pumping motion moves over to the other speaker. If that happens, then the source is the problem.
I have a few recordings that do have subsonics. One in particular is a CPE Bach recording in which the orchestra and audience were apparently on a wooden stage or platform along with the microphones and you can hear the subsonic rumble from people moving around. That's just one example. Another cause of this in the old days was from warped LP records, or turntables that were poorly isolated. The pumping motion of the woofers was very obvious.