I just installed Amazon Music windows app and clicking on the quality icon got this - be interesting to see if setting your audio device as optical vs HDMI changes the "device capability"
The motivation for doing so was to compare youtube vs Amazon music, my sample track was "Goodbye Horses" by Q Lazzarus.
What is interesting to me is AM has several versions and it's as if they're "mixed" differently. The "ultra HD" version opens with a booming bass tone, almost like the THX effect, whereas the HD version opens differently
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And Pink Floyd DSOTM says this, which suggests something in my chain is a bottleneck at 48 khz
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EDIT: going into Denon HDMI properties and adjusting some settings "fixed" it
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OK this is interesting... I don't get like "onkyo avr".
What I see is nvidia rtx3080 audio out or something like that - The amp doesn't show as a sound output at all, did you rename yours? If your sound is going out from gpu it should be gpu name?
I will have a play with amazon music desktop app or tidal this wekeend and let you know what I see so can compare.
However in my scenario I don't think the capability list will be going by what the optical cable can ACTUALLY DO, because IIRC correctly one of the only downsides to using a toslink is it doesn't talk to other parts, it doesn't say "I'm hdmi, I can do xxxxmbps @xxxbit".
It might be taking that number from the windows settings above.