I went to an audio store one day trying out a headphone amplifier. After some time, the seller told me about his USB cable shootout and he indeed had heard differences between a normal usb cable and a better one. He plugged in a Cardas USB cable between my laptop and the headphone amplifier along with a different power cable to the same headphone amplifier. Surely enough, I heard a difference too.
 
To my ears, the sound had become much brighter. Listening to Belle and Sebastian's "Like Dylan in the Movies," I heard much better articulation and isolation between each instrument. But where do the improvements specifically come from? Articulation from the power supply cable? Isolation from the usb cable? Both improvements from USB cable?