You didn't say what the source was and/or what connection methods used but most, if not all of the hdmi equipped Denon units like my 2807 will apply PLII processing to 5.1 sources to "create" the 2 additional ch's when using hdmi, digital, and 2ch. analog connections. Most of the other major brand avr's should do something similar.
Key word being HDMI for multi channel as very few avr's will post process any signal source coming in through the analog multi-channel input. Some Sherwood NewCastle gear will digitize the analog multi channel input and do minimal post-processing such as x-over, time alignment, etc. but they still won't "add" ch's via PLII.
Basically if your going to have to use the multi-channel analog connections your probably going to be limited to 5.1 discreet. Some avr's have 7.1 inputs but most source players with multi-channel analog outs only have outputs for 5.1. In these setups you can always just split the surrounds and send the same signal to both the side and rear surrounds...not discreet but it still sounds quite good. I do that for my sacd and dvd-a source and it would also work for hd audio codecs in movies as well.