Streamed music sounds awful in my system and I don't know why ....

Get the PC out of the playback chain.

Great system but it appears the weakest link is Windows Media Player. Have you tried a dedicated streamer designed and optimized for digital streaming? Couple of questions,

Are you a Roon subscriber and what streaming service you’re using? 

You need a real high end streamer. Your other equipment is excellent. I used to run a Moon 650 DAC / CD player. I streamed from PCs and MACs for years to get nowhere near CD quality until I bought a high quality streamer.

My first was a Auralic Aries G2. But I recommend an Aurender. I would recommend an Aurender streamer of comparable or greater value to your DAC. So my first choice would be a Aurender W20SE or if you want to conserve money an N20 (a compromise). But the Moon 780 is very revealing… which is why your streaming does not sound great. You have a $500 player attached to a $17K DAC. Your DAC deserves a W20SE.

Go buy a Simaudio Mind2 and be done.

For the moment, basically everyone agrees...that never happens around here. 

Yes, get a good streamer...stream Qobuz or both Qobuz & Tidal. Don't bother with Spotify, Apple. If user experience is important to you seriously consider Roon before choosing a streamer. Watch this for a good explanation. 

Hans Beekhuyzen Roon Review

Cheers,

Spencer