Optical signals are better technically as they do not change the data at all (coax transfers change the amplitube but its still digital so it shouldnt really matter!). What you want to look at is the end product. Is the Digital to Analog converter on your Amplifier better than whatever it is you are going from?
It normally is, so u should use it if u can (and its easier, 1 cable!).
Cable etc is normally only stereo (with some feeds being dolby-pro logic a possiblilty as they aer encoded into an analog L/r stereo signal) So it aint a benifit really. But if ur dbox is playing AC3 or anything like that, then its pronbably better to use the optical right into ur AV amp that should read it as that and give u 5.1 surround from a divx etc.
Soundblaster live's have no hardware decode of 5.1 or anything like that thru optical, they only do pass thru, so if u output a DTS DVD from the optical out INTO an AV AMP or decoder it will be 5.1 but u wont be able to record it. only a standard stereo file/stream. (u can actually encode a stereo file that os full DTS 5.1 24 that will play back on a cd player but it needs to go into a DTS decoder to play rights very cool but hey not the point!)
Kinda went off on a tangent there but hoped this helped a bit.
