You will need to first make sure your Fractal and Saffire are set at the same bit depth...spdif is pretty dumb-downed it is a simple digital audio protocol (spdif=consumer, AES is professional not much difference other than header info on first bit), but it will not play nice if the sample rates are different.
Next make sure your Saffire digital is enabled in Sonars input device...(I use Lynx and its a helluva gamble deciding Which channel is the AES digital in/spdif since there are like 16, I typically open my Lynx interface page, play some digital and see which channel is popping then map that one in Sonar)...
Beyond that I am of no help, I have used my Behringer V-amp like this, but tend to avoid doing it because the converters in my DAW and my outbaord tube preamps are better...
I HAVE done this from one DAW to another via spdif when sending over Kontakt samples/etc especially when I wanted to avoid inline noise and re-amping...straight in digital to the DAW IS cleaner once you set it up...you are actually sending in preconverted digital info ONLY, which is kinda sweet, the only noise is what you put into the first converter.