Best Subwoofer for my 2.1 setup

[allstar780]

This isn't another of those threads looking for recommendations for equipment....

Okay I lied.... I have a stereo setup (for DVD/CD music, no TV) in my bedroom using B&W 601 Bookshelf speakers and the Onkyo TX-8555 stereo receiver... the bookshelves have quite a bit of bass output and are very punchy in the "mid-bass" kind of range, but obviously lack the oomph for the lower bass frequencies.

I'm looking for a small powered sub (10 inches) for a decent price (under $500) that will rumble off very low frequencies... none of this 45Hz crap... I need it to go at least as low as 25Hz... I've been looking at many options, but as I'm sure many people will come to the thread and shout out recommendations without reading anything but the title, I won't bother to list anything.

Thanks in advance for any advice!

edit: also the box can't be huge as i have limited floor space