example:
um18 subwoofer in 4 cubic foot sealed box. red line. f3 is 28.5hz and qts=0.886
target is black line. this is what you want after the linkwitz transform. this is completely arbitrary for the example. it is qts=0.707, f3=17hz.
here is the raw sub again (red) with an ideal q=0.707 rolloff (blue). the first step is to try to match the blue line.
this can be done with a single parametric eq on the corner of the sub. in this case, we will use parametric eq: freq=35hz, q=0.88, gain= -1.9db.
once that filter is entered, the red line will match perfectly the blue line.
then we simply apply the shelf filter. in this case, i used +12db to extend the rolloff point a full octave from 34hz down to 17hz. you are done! perfect match to target response!
if for some reason, you wanted a final qts that was different from 17hz, then a parametric eq could be used to change it (essentially doing the opposite of what was done in step 1).