The Rythmik Audio website's FAQ section that I've been re-reading since I bought my F12G sub seems excellent to me. One fine example:
( @soix )
"Cone excursion goes up 4x for every octave lower in frequency. So 40hz needs 4x more excursion than at 80hz. And large cone excursion increases distortion and in particular "intermodulation distortion" (higher frequency (small excursion) signal is modulated by low frequency (large excursion)). The correct way to address this is put a high pass filter (HPF) on front speakers in order to reduce the cone excursion. This issue is particularly bad for ported front speakers as you may notice with full range signal, woofer in ported speakers have far more excursion than that in sealed front speakers. With the cone excursion reduced, the distortion from the front speakers is also reduced and the sound becomes more dynamic and coherent."