I suspect your in a null where your taking REW measurements although they look good. The fact this PB2000 is also making the same cone flapping sound (as you describe) like the pair of SB1000's would indicate you are over driving this sub as well. Upping the avr trim will not help. MLP and or sub location needs to be changed,, or if not in a null, your a basshead and need two large sealed 18" subs. This sub should be pounding in that size sealed room.
-Funk 21.0LX Master/Slave combo 4800 watts RMS/9600watts (peak)-
-Chane A-2.4 L/C/R
-Chane A-1.4 side/rear surrounds.