you must run usb to get the Best out of a dac only usb ofers asyncronous data transfer which cits down on zransmittteed jitterActually this is not entirely true. While most do, some DACs do not use Asynchronous USB, but isosynchronous; also not all DACs sound better from a USB connection depending on how the grounding and power rails for the USB interface have been implemented with the rest of the DAC board, not to mention variations in the source of the USB stream.
There are plenty of DACs with SPDIF/BNC/AES inputs, which when matched with a properly clocked (low-jitter) SPDIF output, will perform better with the same files compared to a USB delivery. It really depends on the DAC and application.