For $500, you can get a nice set of monitors with 8" woofers that has decent bass response. Look at the Mackie MR8 mk2 or Behringer 2031a monitors, and also the Rokit 8 Monitors. Are they going to deep as deep as a setup with a sub- no, but they will have very tight and controlled bass within their range, much more so than any equivalently priced setup with a subwoofer. And they mids and high will almost certainly be better in that they will be more neutral with good off-axis response. It also spares the expense of buying a receiver. No 5.1 setup is going to sound as good, especially when you throw in the cost of a receiver. Quality over quantity.