The USB audio standard transmits an analog step signal over the USB cable that is then reconstructed into a digital signal by the USB receiving chip in the DAC. This is different from the USB mass storage signal that incorporates error correction. The USB audio standard relies on the endpoint (the DAC in most cases) to reconstruct the data of the analogue step signal into 0s and 1s in real time with no capacity to re-check or correct a garbled signal (as would be the case with moving data over USB to a hard drive). The USB Audio standard is over 130 pages, so this is just a simplification to point out that USB Audio is not moving as 0s and 1s with error correction, so the quality of the signal can be affected by cable length, noise on the 5v line in the cable, external RF interference, cable geometry, and the quality of the USB receiver chip in the DAC.
Below is an image of the degradation in the USB audio analog step signal moving through a USB cable. What goes in is not what comes out. If the audio system is highly resolving and well setup, then critical listening can and does reveal differences in USB cables and in the quality of USB receiver chips in the DAC. This means it is something that each person will need to evaluate for himself. There's no wrong answer, but it is a real variable that will make a difference for valid reasons, depending on the listener and setup.
