The only way a USB cable can make the music sound different is if it allows bits to be corrupted, i.e. cable doesn't meet spec. Serial transmission (serdes, 8b10b encoding)is a totally different world compared to S/PDIF. It's quite easy to see if a USB cable is "bad". Just connect a portable USB drive and copy files back and forth and verify the data is intact (checksums, zip file integrity, etc). If this type of data transmission allowed for bits to be flipped, we'd be having a real hard time posting here.
