If you can't find one of the subs already recommended at a price you can afford, the Mirage Omni S8 is a nice little sub for the money. Retail is $350 but if you look around online you can find one much cheaper.
Great -Small- Cheap subwoofer ?
I have a really sweet and very compact system consisting of a pair of the new Role Audio Sampan FTLs, a Winsome digital amp and an Oppo 980h- for the bread it is amazing and even the low end of these small speakers is not MIA- Nonetheless I think there may be some small(8 or 10 inch )subs out there that could supply a bit more foundation(under or around $300) and I'll be grateful for any input.