Check out partsexpress.com, they have got nice low-budget subs (diy)
I've got a REL upstairs in my hobbyroom, and i am using a 15" 800watts rms powered sub in the living room..
If you want to build your own subwoofer, i can help you with designing the enclosure.
Before i was into RC i was into building speakers. Stepped away from it since it took too much of my time.. Now here i am...
phase is very important in a sub, and so is the cross-over. Don't use two cross-overs on one sub; if the amp has one and the sub, dissable one of both; they interfere and can cause nasty phase problems.
Unlike what most companies make you want to believe, a sub needs to be in the center of the speakers, otherwise it's pulling away the entire stereo image. odd but true.
Speaking of nice subs.. I used to have 360 liter large subs, playing 20hz at a 104db/1m/1w efficiency. (at 10W they where playing as loud as most subs could do at maximum power) the FS of the speaker in that cabinet was 14 hz. at 2X800W rms they could help you breath.. it was the most impressive bass i ever had.