ah, i see where all the confusion is now...i think.
here is how i understand it:
the BLUE line represents the frequency response of the AMP by itself.
q=2, second order highpass at 30hz.
the AMP is down -3db at 20hz relative to where it is at 100hz. that is probably where the confusion is coming from as we normally equate the -3db point with the crossover point for a second order crossover.
the RED line is an arbitrary sealed subwoofer, this one happens to be the alpine swr in 1.5 cubes.
it has the characteristic second order rolloff with an f3 around 40hz.
now COMBINE the response of the amp (the BLUE line) with the response of the the driver (the RED line) and you get the net effect which is the YELLOW line.
the net effect is a -3db point of around 28hz and a very steep 4th order slope below that.
now let's compare that to a 2nd order butterworth high pass at 19hz and +6db of boost at 30hz with a 1.4 q. that is the GREEN line.
i was kind of surprised, but the YELLOW and GREEN don't appear to be as different as it might have seemed at first blush.