Generally, I mention power line adapter only as a last resort - because it affects the power, which in turn may affect the sound quality of audio equipment (according to some people).
If you use a thumb drive, the data flow is from the USB. The Ethernet hardware port is not involved in the flow of music data, but it is still used for communication with the app. As you've found out and validated many others' experiences, using a general purpose computer as a source with USB connection to the USB DAC is not ideal, because the computer EMI/RFI noise passes through the USB cable to affect the sound quality of the DAC. This reminds me of a recent report of replacing MacBook Pro by a dedicated network transport:
http://www.audioshark.org/showthread.php?t=10568&page=2&p=181882&viewfull=1#post181882