Hmmm... I don't know much about how the USB audio input is actually implemented on the 990 - it may be a driver issue between your PC and the 990, or an issue with the USB ports on your PC, or something else entirely - so it's hard for me to say just what originally led to your USB input's poor audio performance. Clearly it was somewhere in the USB chain (PC's USB controller, USB drivers of some sort, or the 990's USB input)...