If you want serious subs. EAW or JBL professional make some nice double 18's, but then you need the room for them. EAW makes this double 12 that is phenomenal , but not cheap, and you will need their processor
[Link: eaw.com]

An excellent choice that is very reasonably priced and will provide limitless bottom end in a home theatre is this one from Mackie
[Link: mackie.com]


For really decent results use two of these
[Link: miragespeakers.com]