Sounds like the rear channels are set to the bi-amp mode as the others have said, check the settings

page 19 and 23 of the manual kind of explain what to do:

http://us.marantz.com/us/Support/Pages/Manuals.aspx


Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70