My opinion is that to get anything better, you would have to spend $300-$400 on a pair of L/R speakers and run a 2.0 setup, building your setup over time. If you are looking for a full surround setup 5.0 or 5.1 for that price range, the S606 is hard to beat. I just can't think of another setup from any manufacturer in that price range that competes.
There are lots of great bookshelf speakers in the $300 to $400 price range from Hsu, Energy, PSB, Ascend Acoustics, NHT, etc. The drawback is you would not have the satisfaction of surround sound now. It would be something you would build over time. It would be a nicer setup, but obviously you would be sinking a lot more money into it.