I have a Yamaha RX-V3800 with a 7.1 setup.
Here's what I posted about this on AVS back in December:
"This had me going a bit nuts for days. Go into the setup menu, and look at all entries under sound, stereo/surround, extended surround. I don't remember all of them, but go through the whole menu and you'll find them. The settings your looking for have two options: auto and last. They default to auto; set them to last. There's also one entry where you select which decoder you want to use; select PLIIx.
Now, to get sound from the rear surrounds, press the "movie" button on the remote (#8). You should now get sound from the rears. You can keep pressing that button to cycle through the different PLIIx CinemaDSP modes (adventure, sci-fi, etc.). As you do this, the receiver will display the different names.
This applies when the receiver's display shows DD, DD+, DTS, TrueHD soundtracks when in surround decode mode (button #10 on the remote).
If you're watching a tv broadcast that has a legacy 5.1 surroundtrack, the 3800 will display PLIIx, and give you full 7.1 when in surround decode mode (again, button #10).
Keep in mind this applies to the 3800. I'm not familiar with your receiver, but hopefully this info will give you a starting point as to what to look for.
HTH
Jack