I MAY BE wrong, though I'm gonna assume I'm correct based on my testing a few days ago:
Forst and foremost, update the drivers and software of the G6 to the latest versions which came out yesterday. This MAY have actually enabled multi-channel out through the spdif. It may have happened before, but do it to be safe.
If you wanna pass JUST 5.1/7.1 so that your external device reads the signal as multi channel audio (no processing), make sure you have the G6 software under setup on Spdif direct, and also, make sure bit and sampling rate is set to at the most 24bit/48khz. (this is changed within the windows audio panel itself i.e. clicking on the speaker icon on the taskbar and going into those properties for the G6.)
The G6 is capable of 32 bit/and WELL ABOVE 48khz, but if you want multi channel audio, you must NOT go higher than 48khz, as that is the limit for 5 to 7 speakers. This is what you want if you just want your headset to do its OWN virtual surround. This is how I'm getting Dolby Headphone to work properly on my Mixamp 5.8 that is attached to the G6.
I'll try and re-test in the morning and re-edit this to be more correct, as I'm going off memory at the moment.
I see the Z906 is a speaker system, so you want the G6 to be in spdif direct mode. You do NOT want the G6's SBX VSS to be enabled on the G6 for the Z906. You just need it to send a straight 5.1/7.1 signal. Again, go to Windows audio panel, and set the bit depth/sampling rate to 24bit /48khz.
Please let me know how it turns out.
In the bolded statements above, I know you're referring to output over SPDIF-Direct. I just want to confirm, though, that when using the G6 and SBX, either with headphones directly connected to the G6, or the SBX mix being piped out via the line out, will 32bit/96kHz be ok to use with consoles/PC, or is 24bit/48kHz a better choice in those situations too?
Thanks.