Using digital out and analog in precludes you from running a soundcard calibration routine. So, if the intrinsic response of the line-in port is poor, you'll have no method of compensating (assuming that REW recognizes the line-in in the first place). AN analog line-in and line-out allows for a soundcard calibration routine to be created that renders the card effectively flat so its response is removed from the measurements.
brucek
