USB will handle 24/96 HD audio and you would not appreciate the definition over that until you get expensive.
Check that the NAS itself does not have player software on it that will play out via your DAC (via IP address). Most common NAS servers Synology, QNAP etc will have 2 or 3 open source players available. You will need a PC or iPad to access the NAS in any case.
Why do you feel the need to move files from the NAS? You said you were using JRiver on a PC. This can access the NAS and direct the output to the DAC. You will be hard pressed to find a pure DAC with ethernet on it.