optical isn't the best connection. you might want to try an optical to coaxial converter. the sabre DAC chip is tempermental, and i've read of a couple people having trouble with optical connections if not with the yulong, with another sabre based DAC. in general, optical sucks because it's more jitter prone. i couldn't get my TV's optical out to work at all and was annoyed that it doesn't have a coax. i think there are optical to coax converters out there very cheap. i seem to recall one particular one being popular but didn't pay much attention as tosilink only is never an option for any piece of gear for me.
I think I'm going to try this: http://www.radioshack.com/product/index.jsp?productId=12195987
Hopefully, this will at least show that the D18 works correctly. If I like the sound signature of the D18, perhaps I can get something higher quality. (Or maybe the Yulong U18 USB converter)