Downloaded files have the potential to be even better sounding than CDs because they can be 24 bit and higher than CD sample rates (> 44.1 kHz). It really comes down to the care taken in the mastering if that potential is achieved...
Rlwainwright is correct when saying "potentially" better sounding. With over 250 album titles downloaded from HD Tracks, some of the recordings are astonishing and others not so much. In answer to your original question, SACD/CD Playback as compared to HiRez files of the same. My Esoteric K-03 CD/SACD is slightly better then playback via K-03 Async USB or Bel Canto RefLink USB Converter to K-03 SPDIF Input.