Rel Q108! these are very nice subs and even held up to my use of two of them with my Wilson Duettes. I no longer have either, but the price for one of these subs is right at your price range. Easy to work with, easy to integrate and nice and small. You could find a larger sub and probably for cheaper (right at Best Buy), but I doubt it will be anywhere close to as good as the Rel.
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.