If you're wanting to have only stereo inpot without any DSP, why even have Surround enabled in SBX Pro?I know, thread revival but I just ran into this problem with my x570 (my x470 was fine). Seems there is a solution for the time being in SBZ Switcher (3rd party).
Get it from https://sourceforge.net/projects/sbzswitcher/
I've not tried it yet but seems to work for some people and I'll mess around with it later or tomorrow as having to set to Stereo every boot might start get annoying.
And anyway after disabling Surround and other processing in SB Z's control software it doesn't matter if Windows has 5.1 in its speaker configuration.
Stereo input signal will be kept as such.