To be honest i dont think any of the subwoofers mentioned below/above will float your boat. The Bk no matter how good it is will never extend past what your speakers can already do.
Yes you will get greater head room and firmer and puncher bass from the whole system but nothing any deeper. If you want deep for that budget and want to stay farly room friendly you should build your own (if your feeling handy) but to get any solid extension from retail subs you need move your budget up another 2-300 or so and look into SVS with there PB and SB models. They are are the best bang for buck with great magnet structures and amps. But if you dont have pre outs and need to use high level look at the BK 400 and monolith.
But remember a subwoofer will only go as deep as your room will allow. Which is why i dont really touch them for muisc. unless you get one with a eq inside like a seismic 110 , which i had the pleasure of owning. But there quite a few bob. Or get an antimode.
But do only buy once (which is great advice above) so save for a bit longer and get something a bit more specail!