I've tried running both Toslink and coax digital output from my cable box to my DAC, and it was a horrific failure both ways. I tried this with both a CI Audio VDA-2 and a Wadia 781i, and with both DACs, I got this annoying loud fluttering sound that would not go away. Moreover, I tried this with two different cable boxes (both the same model) in two different locations, and had the same results. I guess Comcast doesn't want me to use their digital audio.
It's probably worth a try, but there's a chance that it might not work.