Ok I found this reading it all I take from it is it simply adjust the curve between the subsonic filter and the low pass what do you guys think?
Q-Factor Control of Lowpass Filter: QFC allows a control of the lowpass filter slope; basically turning the QFC potentiometer will change the damping quality. Depending on the individual requirements, it is possible to give the subwoofer a "slimmer" or a "fatter" sound. [the setting is from 0.5 to 1.2]