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

Well you have found what  I have found it is plain and simple very hard to get a streamer close to a high quality  cd player. From what I have heard of streaming  and the cd player you have good luck finding something  as good. 

The Mind2 interface and streamer in your 780 are easy to use. Down load the app on your IPad and connect your 790 to your network wi-fi or Ethernet.

You’re getting a wide variety of advice here, some good and some bad.

with the price of the rest of your system, it would be a gigantic waste to keep an ordinary PC in the chain.

 I’ve found that Roon running on a separate dedicated server (like a Nucleus or NUC) feeding the player through Ethernet and the player feeding the DAC through USB is best.  AND turn off the Roon DSP and volume control in the Roon settings.  Makes a huge difference.

A quality Ethernet cable and USB cable help a lot, too.

@dr-john You can use a computer to stream with amazing quality. The easier way is to buy a dedicated audio server (really a computer) as mentioned by many others on this thread and be done with. If you are a bit more technically oriented, then you can easily get a CHEAP computer based streaming system setup as I described previously (I did not dive deep in the description).

Most people on Audiogon go the dedicated audio server way and that is a great choice for most. Though this is not the way I do it.

BTW - I was also curious about the elephant in the room. Why use the Windows media server app if you have MIND2?

As everyone else has said, dumpthe PC, get a quality streamer…..even an iFi or Node is better than what you have