Your PC speakers won't be able to make a sound like an amp, no matter what you do. It's not the sound card, it's just that your average computer speakers without a sub push 5 watts or so and aren't made to make a lot of bass anyway. Even with a sub you're only getting 20 to 50 watts (for a small system), and that's still going through speakers that aren't meant to produce the frequencies you need en masse. I could see a fancier system doing an OK job, but it's still not optimized to put out the sound you expect.