Not yet a purchase but I am itching to add another sub to my HT. Was thinking of something like the following:

Shiva-X 12" subwoofer in a ~120L sealed box, Bash 500 watt plate amp, and Linkwitz Transform circuit. Only thing stopping me right now is lack of funding. Then again, I have plasterboard already rattling with the modest sub system I have now. I'll have to sort that out foist. Also extremely annoyed by the lack of good movies lately.