1. I had similar problem with my desktop PC and DAC inside amplifier (d3020v2) connected over Toslink. Initially I blamed the DAC, because when I used analog input, the sound was fine.
I would guess it sounds like this:
Later I found out that I used bad driver for the PC soundcard (it manages Toslink output), that there is an audiolimiter when gain is above -0,14dB and that AUX and OPT outputs are unbalanced and OPT is about -4dB compared to AUX. When I fixed all this, OPT and AUX sound exactly the same since.
2. I recently purchased bluray of japanese movie Ghost in the Shell, 4k+FHD package.
I found out that PowerDVD20 is automatically switching from DTS-HD MA to AC3.
It does so, because its trying to bistream all 6 channels at once, and Toslink supposedly does not have enough bandwidth for lossless audio.
Workaround for me - software downmix to stereo (in 32bit float 48KHz) and then sent to Toslink (as 24bit padded, 48Khz stereo)