Originally Posted by SanchoPanza
also, Dolby Digital Plus 6.1 AND TrueHD 6.1 automatically adds the 7th channel...
That's the only way you would want it to be, and it's the way ANY 7ch receiver will work.
Think about it -- if you have TWO back surrounds, why would you only want ONE of them making noise? Which would it be, the left one or the right one? Either one would be "wrong" because the (theoretically) central back surround signal would be off to the left or right.
What happens with a 6.1 track is that the mono back surround channel is played in dual-mono by the two back surrounds. This is how many 6.1 systems were intended to be used way back in the day, because of a localization artifact where a speaker directly behind your head can trick your ears/brain into thinking a sound directly behind you is actually in front of you.
Even before full 7.1 with stereo back surrounds was an option, many experts recommended using TWO back surrounds for 6.1 systems specifically to avoid this artifact.
Many modern receivers will allow you to override the 6.1 signal and use something like PLIIx to instead extract stereo back surrounds rather than the mono back surround. But either way you WANT it playing from both speakers.