It looks like new gen motherboard can change the audio jack functionality, so if your setup is 5.1, you're going to need to use both the jack at the back and one of the jack on front panel, cable routing is going to be a mess, just for the sake of sa...

I'm in the same shoes, using Logitech z906. Looks like a cost down move by Asus, probably people using 5.1 with windows are a niche group. Weird thing is they stated new z890 supports Dolby Atmos decoding, yet removed 5.1 analog out.