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

Kill your PC, almost anything will be an improvement. I’m only one step ahead of you. I ran a stripped and optimized Windows laptop streaming Tidal. I upgraded the USB cables 3 times and had a noticeable difference. I recently switched the laptop for a Pro Ject Streambox Ultra II, it was a major improvement. Now consider that the people above me have listed some significantly better equipment and they are  experiencing better results than me ! Small Green computer, Audio Troy are solid people that know their stuff. Also in retrospect I have a $2000 Shiit DAC and the Project is $800. Then ad 1 audiophile power cable for the DAC and a linear power supply for the Project and a $300 USB cable , your at $4K  ! If I had a do over I’d get a complete unit in the $5K range. FWIW, I have a nice Thorens TT and a good cartridge, and I enjoy the SQ of streamed music as much, and it’s a lot more convenient. Happy Listening, Mike B. 

@OP

As already told by many, obviously your issue is streaming from your PC, you need a separate streamer. However you don't need to pay 5k or more for it as many recommend. Take a look at my streaming setup in my system. Not counting the DAC, I have only around 2k in it including cables. And it sounds just great. As good as my vinyl rig and cd player, just a slightly different presentation for each. However I did spend a good sum of money building the proper listening room, so I don't have to buy top of the line equipment or accessories to make up for the shortcomings of the listening environment. Foundation first.

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.