I use a Bag End Infrasub 18. Very nice reviews. Wish i had room for two so i could have one next to each speaker. Excellent IMHO. Don't know if this is out of your price range but i saw a very nice one used a week or so ago for around $900. A very unique and great sounding sub. Good luck on the down low.
Best Big Sub? or duals?
I just moved and my HT is a 25 x 30' room in the basement with concreat floors. My current sub a vandersteen is to musical and just not big enought to have any effects, I also have a 15" velodyne that just about cuts it but doesn't really shake the room and the sweet pot is to narrow, probably because of lack of power my old house this did great in. I am using all vandersteen speakers so they are really fast I need a good fast sub or two that will shake the room. Any sugestions cost is not a big issue just want to do it right and don't want to spend more than I have to.