The link you gave for the speakers had the solution on it: Klipsch Synergy SUB-12
Other options:Klipsch Sub-12HG Synergy SeriesKlipsch SUB10 10"
Any decent powered sub that will get down into the 30-40Hz range should do. Unless you know what you are looking for, stay with the name brands. You can get better and/or cheaper going no name, but you have to have some idea of what the specs mean and what to look for: over-rated amps and undersized magnets, etc. If you want to stay with Klipsch, I'd probably go with the Sub-10 as it will be a good balance between cost and oomph.
Oh, how big a room?