-3dB @15hz in the world of home theater would be rather limiting. Even to pipe organ recordings.16hz would be less than ideal, and 8hz notes would be very weak,... But everyone has their respective view and listening preferences.
The range of a true "sub" (meaning below the 20hz hearing frequency limit) ,should go down into the single digits to be able to reproduce all material presented to it. 90% of them are limited to 20-100hz, it's what consumers have accepted. I always have to restrain from telling someone that their system didn't come close to reproducing live levels of DEEP bass when asked, " what did you think"?. Especially when you see that they have enormous amounts of money in their system. At demos I usually give a warning that I am not responsible for bottoming out drivers, ripping spiders off their landings, clipping their amps, burning up voice coils, and blowing fuses (all of these have happened before ).
I am having a good day physically and want to thank all who have participated in this fun thread.
In response to RonR's question about design considerations by manufacturers, at my company (I own Aluminous Audio) we adhere to the multiple subwoofer system concept. Many reasons for that with one extremely good one...smooth, musical sounding bass throughout the room. When set up with 4 subwoofers the soundstage gains a large degree of depth and clarity.
I only use two subwoofers and have the same experience. One is ported custom design(2 scanspeak 16's) and the other is a Velodyne FSR18,both integrate very well and are seamless. The dual sub rolls of at 40 and the FSR18 at 80Hz. The room is 14x20 but opens into about a 1100 sqft area. I listen at about 9:30 to 10 o'clock on my volume control and the bass is clean powerful and very musical. One thing is that drum kicks are centered perfectly between my satellites and have excellent punch. On pipe organ music the bass can vibrate my whole two story structure. On Fanfare for a common man the Tympani's are very articulate and resemble bombs going off.
I don't know why but the overall all quality has improved greatly with both subs as the music takes on a effortless quality and certainly fills the space. I have no desire to listen at a higher level,in fact I can listen at lower levels and still reproduce the music as it was meant to sound,but there is a optimum level for each recording.