ROG Strix Z790-F (A step down in audio quality from my $300 2015 Crosshair V Formula-Z it seems.)
When you have 7.1 speakers and pay $500 for a motherboard you expect it to do something as simple as have the Speaker Fill audio enhancement enabled so you can listen to your streamed music and 2 channel video on all your horns. How do you unhide the option to enable speaker fill in the enhancement tab of the driver settings. This is a standard and basic feature that any 7.1 sound capable board is supposed to have, hiding it from me is unacceptable. Even with local mp3 content turning on SS3 upmix sounds horrible its not a simple stereo speaker fill. This is Windows driver 101 knowledge ASUS come on get with the program.
I have gone over your description and what you describe checks.
When I enable UpMix/Surround on my system it does require room correction to make it normal. There was a suggestion, in another thread, regarding the Amplify level being turned down to Performance to cut down on distortion. I had been listening to audio on my system for quite a while with the option enabled and never noticed any distortion.
The poster that should be able to answer your question would be @MoKiChU if he has not responded prior. It seems there was a security situation with the Speaker Fill and the option was removed.
I hope you get it worked out.
There is no security issue with the Speaker Fill enhancement, the issue is lack of attention to detail that differentiates between a great product and a mediocre product. This tier of ASUS motherboards used to have great audio; in its current state the motherboard audio is just mediocre.