Don't know if anyone's mentioned this yet, but to extract more bass, I just connect my bass amp to the sub out on my hifi.
ive never had much experience of subwoofers but I guess that a bass amp is going to work best, and it takes some of the load from the hifi amp if the amount of heat generated is anything to go by.
In most cases the impedances should match, if you use the line in connection on your bass amp.
should make quite a difference, though you may still struggle to hear the bass on metallicas '...and justice for all', lol!
great topic, btw.