Yesterday I made a A/B comparison between the two input with the help of my wife to avoid any subjective influence

Same song, Barbr Jung from Linn Records, 16/44

on the usb input I have a flac played by foobar
on the toslink input a burned CD played by an Onkyo DVD player

Well, I did expect to find the CD a loosy support, but it's a matter of fact, the CD sound (slighly) better; my wife and me agreed the most noticeably difference was surprisingly not on the highs, but on the bass definition

from the usb it was slightly blurred and rounded compared to the CD

Now I wanna try to play a WAV instead of a flac (I red somewhere it could make difference)