My Sony 380, 590, PS3 slim all do 5.1DD+ on Netflix Canada. My Oppo 95 is the only one that would do stereo only. I am watching Hellboy II right now in DD plus, and all channels are actually making sound.
So I think it has to be the settings on your slim, AVR, or something to do with the cable but then you said it plays BR fine so that is not so likely. May be do a microprocessor reset on your AVR and the PS3.