As Shady pointed out, If you end up getting a second PB-16 and have similar problems, for kicks and giggles, try running one of the two subs out of phase just to see if it makes any difference. I remember back in the ole car stereo days having to sometimes run a mid/midbass door located driver out of phase (manually or via the headunit).
Don't remember the technical jargon but I guess the two drivers were cancelling each other out when run in phase. You may be experiencing that in your room.