Just played some Wildlands with the HD650 on the GSX1000 to test how it sounds and which volume level it needs. 7.1 active on the GSX1000 and in W10 playback device settings. Volume was on 48 out of 100. I worked fine overall, the VSS was very good and convincing. So you could do it, keep the HD600 and buy the GSX1000.
Ok that is interesting! Nice! Will check out GSX1000 afterall then.