Limits of Sub Woofers

I've heard that the THX logo sound has 16 hz tones. My sub, NHT SuperSub "doubles" when the logo plays. It rolls off >3db below 37 hz. What I'm interested in knowing is what are the practical limits of home sub-woofers, taking into account room boundaries, which ones go measurably really low and what are the frequency extremes of most movie (DVD) soundtracks, i.e., what do we need?
Most movie tracks cut off around 30 hertz. My Velodyne HGS-18 is flat to 15 hertz and quite a bit of output down to 8 hertz. This is in a fairly large room, corner loaded.
Curtaudio, please describe how you determined the Velodyne's low frequency measurement. What test disc, SPL, whatever. I'm curious.
no it dont man das a big pile o notin ya chatin ya unda stand??????