According to the owner's manual, the default setting for the 2808 is 5.1 with the back surround speaker jacks set to Zone 2. On page 31 of your owner's manual, it shows how to assign the internal amps to a 7.1 configuration. I didn't read the whole manual, but I'd suggest assigning it to 7.1 and the rerunning the auto set-up routine.
If you would, please let us know if that solved your issue. Thanks!