I've just registered, because as a new Corsair HS70 user I'm having the same issues. Volume is really low, issue occurs only in PUBG. Need to crank the volume to 100% to play and it's still a bit to low. The changing levels of ingame volume occur as well - but it's more like +- 5% for me.
I've went to HS70 from Logitech's G933 just like MasterKush13 and TheSolidCrimson, having the low volume issue on that headset as well. Previously had the wired G633 which didn't have those issues at all.
I'm considering returning the HS70 as well.
