I have had a lot of experience trying to match various subs at different price points with monitor speakers in small rooms that size. If your floor standers get down into the 30hz area, that should be more than enough bass to fill that room, and provide plenty of "slam" for most types of music. If you insist on more, but are not concerned that much about quality of bass, there are a lot of affordable subs that will give you more "thump", but most of the cheaper ones will create problems with boominess, colorations, lack of detail and mis-matches with your main speakers. This will be most noticeable when playing acoustic music, and often annoying. Better quality, smaller subs such as the mentioned MJ acoustics or REL's would be a safer bet, but may run $500 and up on the used market. These tend to be far less colored, tighter, and faster. What kind of floorstanders and amp are you using now? You may want to look into an amp that proves to be a better match for your speakers.