Both optical and coaxial digital audio use the S/PDIF standard, so when you say the cable box has "regular digital" I assume you mean an optical output and the DVD player has a coaxial output?
You need to make sure your cable box has a setting to output 2 channel PCM digital audio, as most external audio DACs cannot decode a Dolby Digital signal. Most cable boxes have that ability, but there are a few that don't (as well as some TVs that don't).
The preferred USB Asynchronous input is more or less becoming standard, so beyond choosing a DAC that has Asynchronous USB, Optical and Coaxial inputs, the next question would be - is remote control switching of the DAC's inputs a requirement?
Steve