USB vs Disc quality on Oppo

I use an Oppo unit as a transport to my NAD M51 via a Cardas digital cable. My question is given the same digital HD file is there an inherent difference in audion quality between using the USB input on the Oppo vs it reading the bits off the disc?
How about you do the experiment and let us know what you think?
Oh I have. Can't say I hear any difference. What I guess I am really asking is what different digital processes are taking place with the reading of the data prior to sending it out to the DAC - if any.