When you play a stereo source you need to create a surround effect from 2 channels. You would need a surround mode designed to do that. If you have any such modes (Dolby music for example) try them.
When you balance the levels of your speakers under test you are setting them to reproduce what the audio mixer wanted you to hear.
On movies the back channels are used briefly for louder effects and over longer periods for low volume ambient sounds. Some movies won't use them at all.