So, as a G930 user:
1. If the source is stereo, set you HS to stereo, having it mixed/whatever it's called to 7.1 adds a reverb to it.
2. If you use MPCs internal filters for audio you can use the custom channel mapping option in the options menu under internal filters>audio switcher. You can then map channels however you want (including the option to map the same input channel to multiple output channels, which is what you'd need)
3. If you are using an external filter, then open the config of said external filter (in my case LAV Audio config). That's where you'll find mixing options if it has them. In my case, you can't map channels however you want, but there are some mixing options.
4. Since you are using a headset, the drivers are probably mixing everything into a 7.1 format anyway.
Anyway, same as input is probably the best option, unless you know your way around mixing things properly. That way your decoder will give always an output that equals the source (stereo or 5.1) and I believe the mixing is then left to your HS drivers. Ideally it'll at least leave Stereo as Stereo, and probably mix 5.1 into something resembling 7.1 anyway.