I suggest you only have one cable connected at a time if you are comparing input types or cables. Since a lot of the potential difference may be some form of noise transmission by the cable, having both connected at the same time wont remove that variable. The best advice I have heard is to have only one connection at a time connected when you are making a comparison.
BNC and USB from streamer to DAC
So, i’m sure to some this will be a stupid question but: Can i connect both USB and BNC from my streamer to my dac, so i can compare sound, and just switch sources on my dac between the two?