There are some amp assign and speaker terminal connection issues. If the towers are only for surround back purposes, they should be connected to the surround back speaker terminals.
Nothing should be connected to the Front B terminals in this case and the Amp Assign Mode should be at 7.1, not at anything else. No bi-amp A or B or Zone 2 should be selected. Then, 7.1 channel Stereo will be selectable and available for output.