I think I would be looking at the output level from the computer to the receiver.... my guess is that your overdriving your input levels on the receiver.
Have you tried turning down the output level of your sound card by a few Decibels?
People often think of high-wattage and sheer volume when they think of distortion, but frequently it's mis-matched line and preamp levels that are being overdriven. In the consumer world, line-level is a standard that's universal enough to ignore. In the world of professional audio, that's really not the case at all and I'm constantly matching levels because very little has a "fixed" output level. If you're using a computer as a source, I wouldn't be surprised if it was overdriving internally or putting out more than 1 volt at line level....
