Your DAC likely is not able to deal with the higher jitter of an optical interface. Coax has less jitter.audio2 and douglas,
do you mean the optical converter or the optical cable?
In both cases, a similar problem may occur in Ethernet to optical
conversion.
Unlikely the problem is in the DAC (T&A DAC 8) which, in general, delivers a clean and natural reproduction. In general, one straightforwardly assumes that the more direct signal is, the better is the sound (less converters, better ...).
My internet signal comes with a fiber optic cable, are there strong arguments supporting a common opinion that WiFi connection to a streamer would a priory be worse than the cable connection?