USB is a complex digital interface standard, very different from SPDIF, AESEBU or any other audio digital standard. Therefore both ends of the link need a USB tranceiver and therefore both ends of the cable would require termination with a standard USB plug (since it's part of the USB standard to use these plugs).
Also USB runs at a very high data rate and is packetized so if there are DACs that have a USB input (I assume sogood51 knows better than me) then they would have to buffer and reclock the data, meaning that the USB cable itself should have absolutely minimal effect on sound quality (since digital cables effect the sound by their jitter characteristics, and jitter is removed through reclocking).
