PA amps seem to quote 20hz - 20Khz
Our Samson Rubicon 18s Active Sub goes down to 20hz +/-3db
The lowest note on a piano is 27.5hz

But if I look at passive sub woofers, typical lowest frequencies quoted are around 40hz - 50hz.

I'm sure the makers and users of professional audio equipment know what they are doing but I'm curious as to why the (I suppose) best range seems to stop at the low end at what in some cases is an octave above the lowest piano note.