I'm not really sure how much low end you're gonna get with a budget of only $300. the audio engine A2's go about as low as you're gonna get in that price range to my knowledge. My advice would be to save up a bit more and try to set up a way to add a subwoofer. I don't see a sub out on the audio engine's so you may have to wait and save up for a better pair of bookshelves or maybe a little stereo receiver that has a sub output on it.
Also try using the search box and search for computer setups there is some great advice already floating around this website.