At your price point, I would forget about a subwoofer, and just get a receiver or integrated amplifier and a pair of speakers. Otherwise, everything will be too cheap to be very good. If loud is what you want, look at speakers with horns, like Klipsch.
I recommend that you spend about $100 on the receiver and $400 on the speakers, as the speakers will impact the sound far more than the receiver. And if you are willing to spend more, spend the more on better speakers.