Just taking a quick look at the manual, I would agree with mofi's response, however, if you look at features here,
http://musichallaudio.com/old-products-a15-2/
it states that the line out is fixed. This means that the signal is not controlled by the a15.2 volume control. Even though the sub would play, it's volume would only change by using the sub volume control.
If the line out is variable, it will work great as mofi described, and the manual seems to imply it is variable, however the website says fixed. If fixed trying to use a sub is not a good idea.
Looks like you need to contact Music Hall.