CPW,
Like I said, there are headphones that will play down to 20 hz, its just a matter of what level they play at. I don't think Bose is a good example because they don't have any substantial bass below 40 hz on any of there products.
TLs can be very good at producing very deep bass, but they need to be exceedingly long and aren't very realistic in a car. Bose did do a TL of sorts in the 93-95 RSX, but deep bass is not its strong suit.
