Do you have the ability to play REW's signal generator through your system? If you do you might want to try playing around with different frequency tones and adding different amounts of distortion and different orders (most speaker distortion is 2nd and 3rd order) so you can hear for yourself what harmonic distortion sounds like at different levels. The problem with this exercise is it is MUCH easier to hear harmonic distortion when playing tones than it is when music playing.
Regarding some of these music listening examples and LF distortion levels there is a problem which is that most music doesn't have much content below 40 Hz and below 30 Hz it is quite rare. In addition the threshold of hearing at 20 Hz is ~78 dB so even if there is content at 20 Hz unless you are listening quite loud you won't hear it. So listening tests at normal levels where people say they don't hear 50% distortion at 20 Hz doesn't mean anything because usually there is no 20 Hz content to distort and even if there were you would have to be listening very loud to hear it.
Regarding big high performance subs, they really don't make practical sense in most situations but in some situations i.e. high SPL where actual LF content exists they can make a big difference. My frustration is that less capable subs are marketed as "the same thing" as big high performance subs which further muddies the waters because for 90%+ of uses cases they really do sound the same because neither big high performance subs nor "posing less capable subs" are doing much of anything audible especially below 40 Hz.
Dunno.
