Unless you listen almost exclusively to Baroque Organ, I can't imagine much 20hz content. For HT use, where the 'musical' properties of a sub are.....less needed......it may even be a non-starter when listening to 'effects'.
Also, if Crossed-over low enough, that mitigates the effect of 20hz 3rd harmonic. I cross over at between 40/45hz and even my vented HSU produces pretty tight stand up bass. So the amplitude of 3rd/5th harmonic 20hz is pretty well attenuated.
Also, if Crossed-over low enough, that mitigates the effect of 20hz 3rd harmonic. I cross over at between 40/45hz and even my vented HSU produces pretty tight stand up bass. So the amplitude of 3rd/5th harmonic 20hz is pretty well attenuated.