i'm a big fan of passive subs, which i believe have certain audible advantages incident to using the same amplification as the mains. i've had good luck finding very inexpensive units on ebay--i currently own a jamo passive, which is terrific; i've also seen old svs passives pop up occassionally, as well as polks.
Searching for passive subwoofer...
I know that I can get passive one searching pro-audio shops, but is there any high-definition ones available?