Subwoofer suggestions

I have a pair of Pioneer S-1ex's , now looking to add a sub.  Room size is 12x18 fairly well damped.  
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+1 rocray.  I have an SVS SB2000.  It's a sealed box design and not too big, and it reaches down to an honest 20Hz and feels every bit of it when called for.  For $700 I think it's a bargain, but I haven't compared it to the others, which are also excellent.  Best of luck. 
As others have said: HSU, SVS, Rythmik, PowerSoundAudio, and JTR.  
Mom other companies (like REL, JL Audio, etc.) even come close to offering the same price:performance ratio. Not saying they make bad subs, just overpriced.
+2 on SVS SB2000. I have two of them, and together, they greatly reduce subtractive nodes in my room. They go down to 19 Hz, which is low enough for me. With 500 WRMS per sub, there's plenty of clear, note defined, low end grunt.
I suggest the Seaton Submersive HP. Great dual opposing driversub. Goes down to 10Hz.  I have two I use in stereo my
mains full range. With the dual drivers you get almost no cabinet vibration that gets transmitted to your floor.