Z370 Taichi sound issue

I've got a new Z370 Taichi (intel) that's up and running just fine. I'm using the optical output to get the L/R signal to my Edifier S1000DB speakers. Those are working great. No issues. I've also got a Yamaha sub that I'd like to continue using but that's not working at all. The sub is fine. It was working in my previous build but my old board didn't have an optical out. The Z370 Taichi has optical out and 3.5mm output jacks. I've got the sub connected to the sub jack and yes, the sub is on. But no sound. Not even an indication that there's any signal going out from the board to the sub jack. See my screenshot. Is it even possible to use the optical cable for L/R AND use the 3.5mm out for a sub? If so, any advice on what I'm missing here?

Thanks in advance :-)

http://www.oxmyx.com/images/z370/Z370TaichiSoundIssue.jpg

I do not know the answer, but an educated guess is that the subwoofer output won't be active unless the primary L/R outputs are also active.  Because it would be weird to have just a subwoofer plugged in, you know?  Perhaps if you plug something else into the green plug, the subwoofer will fire up?  Maybe even a pair of headphones?