Mark,
In theory, it's 20 Hz, but I believe the line is blurred and varies somewhat from one person to the next. I have a friend (he does the Q&A column for Sound&Vision magazine) who absolutely claims to hear frequencies below 20 Hz. I believe him because he was a colleague and member of the listening panel for years of speaker tests at the NRC in Ottawa, and he's not into BS). The sub in my car has real output below 20 Hz but all I hear is the plastic door panels and other components rattling when the test frequencies go below 20 Hz. My friend actually "hears" them, he says. . .
I'm currently experimenting with the Axiom EP500 sub in my living room, and it's scary at times what I "hear" from it. With pipe organ and orchestral bass drum, there is a powerful reverberance that I really miss when I switch to the old sub (a decent Mirage with two drivers). The old Mirage simply has no output below 27 Hz.
Once you hear/feel genuine output in the sub-20-Hz range, you don't want to do without it. . . You'll also hear weird low-frequency garbage on some live network HD sports broadcasts that have Dolby Digital 5.1. The audio engineers don't realize it's there because the monitors they use simply don't reproduce it. I sometimes reduce the sub level or turn it off when that stuff comes through.
Regards,
