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.
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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.
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Martin Logan Dynamos are very good. has some decent kits, you are better off with DIY kits where you pack the insulation and install the drivers at this price point, the subwoofer boxes will be larger but you will get a really nice sub for the money.
Take a look at Outlaw Audio LFM-2 ... nice little sub. I have one (for HT duty only) to go along with Hsu VTF-2.
Oh yeah ... FWIW, the LFM-2 was designed for Outlaw by Dr. Hsu.
I vote REL. With the high pass input, and careful placement, they integrate perfectly with the main speakers. Sounds like you don't have a sub - rather, like your main speakers just got a whole lot better.
Hsu STF-1, $299. All Hsu subs have 4th order crossovers that go from 30 to 90 Hz -- that steep filter and wide range is a very big deal for dialing in the right match with an assortment of mains and rooms. The STF-1 plays 28 Hz cleanly and with authority, I have mated them easily with many fine speakers. And they are small and light, easy to move around, another big advantage for my needs. 30 day money back evaluation too.

I will second both REL and Hsu.
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.
I have a Klipsch RW 8 sub and really like it for the money. It is musical and fast, not thumpy or muddy at all. I think I paid $250 for it new from an Internet discounter a few years ago. It is very compact and goes down to about 32 Hz. Great for a small room.

The RW 8 is no longer available new, but the RW-10d (10" driver) is for about $500 new. You could probably find one of these 8 or 10 inch driver Klipsch subs used for $300 or less. If you go this route, make sure you get the Klipsch with the "RW" model number, because some of their other subs are less impressive.

The recommendation above for a Hsu sub is also a good one. If you can find a used REL for $300 or under, buy it.
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I heard a demo for the James EMB1000 and thought it was excellent. It might be worth your time to investigate.