USB audio has nothing to do with the cables going to your headrush. USB audio is the audio carried by the USB cable to your computer. If you are referring to audio FROM your computer, TO your AxeFX TO the monitor, then the two cable rule still applies....a single cable out of mono jack cannot carry stereo audio, it's not possible. the reason you are hearing both sides with two cables is that, well, it takes two cables for stereo audio. The HR is still summing to mono, your just hearing both sides summed.