playing too low, might be unloading on making some mechanical noise from loss of cone control. And if it is the last note of put on in decafs, it is ~24hz. could be a combination of many things like port area, tuning, filters on the amp, etc. (i think it is mainly port though). i can play 24 full tilt all day long but my port is like 11 per cube. (i have xl's as well). and i think i have heard what you are hearing but i get it in the 15-20hz range.
edit: for those who dont know, F3 is not the box tuning.
and bottoming out is FUCKING LOUD so you would know.