First of all, please note the "Sticky" at the top of the page requesting that subwoofer qustions be asked in the "Subwoofer Forum".
To answer your question, most PA subs are optimized for high SPL from ~35 Hz and up. This is fine for most music, but for HT applications, they leave a lot on the table. For example, here is a JBL 18" PA sub:
http://www.sweetwater.com/store/detail/PRX518S/
The spec's say:
# Frequency Range: 37 Hz - 140 Hz
# Frequency Response: 52 Hz - 110 Hz
(The difference between "Range" and Response" is that the speaker will have some output to the quoted "range", but it's *usable* output will be limited to the "response".)
Within the Frequency Response of 52 -110 Hz, these "subwoofers" will output 129 dB.
However, for HT applications, you want usable extension down to 20 Hz or below, and most people don't need 129 dB of output.
Craig