The M3 is a DAC that I have retained sort of. I have had multiple people demo it and since there is a little bit of a break-in period, several people have asked to keep mine. My current version has a network card.
I have tested different connections extensively and just had a customer do some testing. USB is almost universally deemed inferior but it is not like it is awful. I prefer the Ethernet connection as I am a Roon user but the USB sounded very good and my initial testing was all from a streamer in via USB.
Ethernet and AES are best inputs but you will not lose much with USB.