Well, first of is the calibration aspect. Then it's the subwoofers themselves and their power amplification. 8" subs aren't really considered subwoofers; they are more like woofers. Personally I wouldn't go smaller than 15" if you really want slam for HT. Then you also consider your source material; like what's on the DVD. There are some good test DVDs or even CDs (for musical reproduction of bass). Fight Club at the end scene is a very good example to try to test your subwoofer. If your room isn't shaking with this DVD go back to the beginning and see what part applies to your dilemma.
good luck
I have a 15" Tempest in an Octagonal Cabinet that is pure magic.

http://gallery.audioreview.com/showp...cat=506&page=2