If you have Choice Xtra, you should be getting the Gamemix. Perhaps I should be getting it too, even with Entertainment.
I'll be upgrading to Choice today or tomorrow, so it will be interesting to see if Gamemix works for me after that.
I just upgraded from Entertainment to Choice, and sure enough, I'm getting the Mix Channel now.
So, the Mix Channel showed up either because my system was refreshed, or because Choice is the minimum package to get the Mix with the EI preview.