My speaker box model also includes a Linkwitz Transform at 20 Hz along with a 4th order low pass at 1,800 Hz in tandem with a 2nd order high pass at 15 Hz produces a near flat frequency as model between 30 Hz to 700 Hz. With -3dB points around 20 Hz and 1 kHz. See below.
|Crossover at 1.8 kHz and maximised bass response|
|Crossover at 1 kHz and slightly reduce bass response to limit Xmax.|
|Closed box with same parameters as above.|
|Passive crossover ported box, no EQ.|
|Passive crossover closed box, no EQ.|