There are several possibilities. Based on my limited experience cables may help fine tune the sound by reducing the brightness but the difference may not be significant enough to completely resolve the sibilance with human voice or glare in the treble.
In my system, the amp and speaker match is usually the culprit with sibilance and brightness. Replacing either the amp or speaker resolved the issue. DACs don’t contribute too much to the brightness in my system. As a matter of fact, one of my DACs (Luxman DA-06) sound slightly warm.
I would look at trying different amps if you like the sound of your Sonus Fabers.