Toslink

The Wattson has an Ethernet input. Why not connect it directly to the network?

Looks like it also supports AirPlay, Tidal Connect and UPnP. You might not need your current streamer with this DAC.

When I switched out an optical cable to SPDIF, I noticed a blacker background, ie the spaces between notes.  Nice improvement.

As I stated, it sounds better with an external 

streamer

The best modern USB, in asynchronous mode, is currently the best consumer link. TOSLINK the worst. It was not always that way so a lot of valid old biases exist.  Can you hear it?  Only your ears can tell.  In a defective system with bad grounding, fiber does break that chain. 

High end fiber cables? Total snake oil IMHO but if you think they are great, then it is your enjoyment ( and money) that counts.  Sharp bends can cause issues so watch the radius. If long runs, like over 10 feet, then glass has less loss than plastic.  The fiber is not the limit, it is the SPDIF interface spec that is limited. We use fiber for 10G Ethernet.  Spend your money on the DAC, Amp, and Speakers. They make far bigger differences. 

I still prefer SPDIF and AES/EBU to USB in general.

A good glass TOSlink cable makes a big difference in my experience.  I've used WW Supernova cables on several occasions with satisfying results.  You just have to try things to see what you like better.