Found this in your manual on p.105:
You cannot use an external amplifie
r for Zone2 when the ZONE OUT/PRE
OUT jacks are set to output front
presence channel audio in “Power Amp Assign” (p.131) in
the “Setup” menu. Also, yo
u cannot use an external
amplifier for Zone3 when the ZONE OUT/PRE OUT jacks are configured to output rear presence channel audio.