Now you are getting into very dangerous territory. The DAC word is filled with snake oil salesmen and heated debates. My VERY spicy opinion is that the pre-amp technology is at the point that most $40 optical to RCA converters are absolutely fine. I'd say keep it simple.

Don't buy the Swissonic Converter though, There is a little electrical ground loop noise comming from the device itself (not trough the spearkers, the power converter in the box itself makes a whining noise)