The directivity modifier you linked works for widening off-axis dispersion of guitar speakers which are used "full range" by throwing away most (around 10 dB) of the on axis level above 2000 Hz.
What application of this "principle" would apply to subwoofers ?
A: Virtually none. Since low frequencies are large (wavelengths larger than source) and the resulting dispersions of subwoofers is nearly 360 degrees it makes little or no sense to try to "widen" the dispersion of subwoofers. You can't get much wider dispersion than 360, eh? As the article states, this modifier has rapidly diminishing results below 1K Hz.