My electronic engineer friend tells me that the various electronics and noisy switch mode power supplies in your home will inject noise into the signal, as well as all the components, from the modem onwards, that the signal passes through. Hence why a good quality network switch, linear power supplies, well-shielded Ethernet cable and well-designed usb can make a difference.
I have tried various usb cables at many price levels and some make no difference to my system, whereas one my friend made himself, makes a big improvement. As someone above said, the key to his USB cable is to not use the 5th wire in the cable, the one that delivers the power, as this is not required in self-powered DACs.