If the 1070 only had mains connected (with or without a sub), the user should have the center, surrounds, and rear surrounds disabled in the 1070's menu. At that point, neither 5 Stereo nor 7 Stereo will be available for use - the 1070 will recognize that the mode isn't applicable to the system configuration and won't offer it as a choice. In fact, I believe that you have to have the center and surrounds both enabled to achieve 5 Stereo (no 3 Stereo or 4 Stereo, in other words) - beyond that, you would have 6 Stereo if a single surround back speaker was configured or 7 Stereo if two surround back speakers were configured.