I am using a Sony 600W 4 channel amp with inputs for front and rear. Since my PC is only 2 channel stereo out I used Y splitters to split Left to front and rear and Right to front and rear. Then I adjusted the output level on the amp itself for front and rear channels. This is done with a small screwdriver on the outside of the amp controls. It's not something you can adjust while you are driving, but hey - fading is normally something you get set right and leave alone. I use Winamp to play tunes and use its equalizer for left versus right balance as it seems you've already found.