Tim --
...I’m fairly certain that 4K Ultra HD Bluray discs and streaming videos don’t contain any audio content below 20 Hz. I’m virtually 100% certain that no commercially available stereo music content, whether issued on LP, CD, SACD or Hi-Res digital file, contains any audio music content below 20 Hz because not one of the numerous individuals, that I’ve asked to identify a single specific example of a stereo music recording with bass below 20 Hz, has been able to do so. I’ve even searched for a single example myself without any luck. Can you name a single example? Anybody reading this thread know of a single example?
This is tangential to your former examples of arguments in the vein of "there’s no stereo information in the bass, neither recorded nor perceived; symmetrical placement of subs is moot (at a not specified cross-over frequency)" etc., and it goes to show what you’d like to feel better about while trying to convince others into believing as well. Sorry to be blunt about this.
Let’s make this clear once and for all: there IS content below 20 Hz en masse certainly as found on Blu-ray’s and UHD’s (look over at highdefdigest.com and their Blu-ray/UHD reviews, where there are occasional bass charts to prove there’re numerous examples of titles with infrasonic information into the single digits), and while I’ve seen no similar documentation on whether CD’s contain information below 20Hz I know of several individuals who have a music collection to strongly benefit from sub systems capable of much below 20Hz reproduction (compared to "just" having honest capabilities down to 20Hz), which is at least indicative of source material actually containing information in these "nether regions." Head over to the AVSForum and see with your own eyes the sub set-ups these people have in their homes, and ask them whether <20Hz reproduction matters. Visit databass.com and ask the same. The answer, I promise you, while be quite unanimous. People may not all agree on the priority of attaining infrasonics (and the compromises potentially involved here), but most won’t deny that frequencies down to ~10Hz (below that to truly matter requires rather massive radiation area and power to make a difference) are perceivable/felt, and can have a big impact on the experience. The proof is IN THE EATING of the pudding, but there’s documentation to back it up as well - should you feel inclined.
My main point, which I believe you likely agree with, is that it makes little sense to have an audio system capable of bass down to 6 Hz if there’s no HT or music content that contains bass that deep. Are you sure you’re not listening and feeling bass that’s going down to 20 Hz and just thinking it’s going down to 6 Hz?
In my room, even bass down to only 20 Hz sounds and feels very deep with powerful impact and realism on both HT and music. I don’t perceive I’m missing a thing.
With my own sub set-up I’ve consciously chosen to forego <20Hz because I favor bass reproduced from horns, tapped horns at that (this requires of you to actually buy into that bass isn’t just bass, be that via distributed arrays or not), and while infrasonics can be had with bass horns they simply end up being ginormous (so, a practical consideration), or with tapped horns in particular it means giving up extension in the upper frequency range, eating away sensitivity (though it’ll stay higher than any typical direct radiation sub) while continuing to have the physical size grow into behemoths when seeking an ever lower tune. When in my tapped horn subs the drivers move just a couple of millimeters with content down to 20Hz, I don’t feel I’m missing anything either - indeed it’s a visceral, awesome experience that shakes the air. I absolutely agree with you on the sufficiency felt here, although it’s not only about mere extension but also, and importantly about how these frequencies are reproduced.
However, I’ve heard what <20Hz can do when reproduced forcefully, and it adds a dimension difficult to describe other than it has emotional impact (as
@jwmorris expressed above) and can also lead to a sense of unease and even intimidation. It makes a difference in particular with Blu-ray’s/UHD’s, and while I’m a movie buff and know of what I’m missing with my choice of sub system, I feel I gain sonically where it matters mostly to my ears, which is from ~20Hz on up.