I was in your exact same situation and budget a good while ago. I ended up going with the Asus Xonar Essense ST (or STX depending on your motherboard) and I paired it up with a 2.1 Corsair setup of SP2500's.
Corsair Gaming Audio Series SP2500 High Power 2.1 232W RMS 8IN Subwoofer PC Speaker System - Corsair - CA-SP211NA ASUS Xonar Essence STX PCI-E1 SPDIF Sound Card 124DB SNR W/ Headphone AMP - ASUS - XONAR ESSENCE STX
This will come to around 500 or so, maybe less if you can price match or find a deal elsewhere. After customizing equalizer and sound settings(on the sound card software) I am extremely happy. The speakers can go VERY loud and come with a LCD device that lets you control volume and bass, as well as pre configured equalizer settings that I leave at default. The sub is very big and produces extremely clean and powerful bass. It's not distorted at all and it WILL shake your walls. FLAC and 320kbps mp3's sound amazing. Consider one thing though, the sound card is meant for 2.1 which is why I paired it up with a 2.1 speaker setup. the ST comes with an add on card that allows it to do 7.1 if you are ever interested in that. (this is an extra you have to buy elsewhere)
I don't think spending another 300 dollars on top of this would give you much of a performance/quality gain so I found this to be perfect for my needs and budget.