When you use the analog ins on the behringer, the signal goes through a A/D conversion then a D/A, plus the D/A in your SB3. In your case, digital out of the SB3 to the behringer would eliminate two of the conversions. I use mine digital in/out with a CIaudio dac. The D/A in the behringer is pretty good, but the CI has a wider soundstage.
My opinion is, the quality of the rest of your equipment deserves better than your analog in/out plan, and maybe better than digital in analog out. Try to borrow a higher quality dac, go dig in/out and see if there's improvement.
Ryan