When I've run into that switch up upon playback, it's usually MC self correcting an issue in order to facilitate playback. The setting is under Options, Audio, Advanced, Auto configure output settings on playback error (Y or N). I always keep it at Yes. When iMC auto configures and changes my settings in order to allow playback, I've tried to set to No and most of the time get no sound.
It's usually caused by your components or possibly computer. Basically something is saying 6 channel playback isn't an option.
I've always had that same issue when playing to my AMP's For some reason, either my Mac or the AMP's. Though I've tried three, different brands, Yamaha 11ch, Sony 5.1 and Sony 7.1. Can't get MC to play anything more than 2 ch stereo. I know one reason is I use Music Cast or Airplay. Neither support must channels.
One work around to get all 6 channels to come through: Try using Room Correction in DSP studio. redirect the missing channels to your front two and see if they come through.
Good Luck!