I can't figure out where that setting is....
Yes, NEVER turn on ECO mode or set the speaker impedance to anything other than 8 ohms or you will seriously current limit the amp section of these receivers. On Yamaha's integrated amps, it is a switch on the back. On this receiver, i think you have to hold the "straight" button and power on the main zone to get into the interface for switching speaker impedance. I have the A-3080, so i'm not sure what it is for the A-1080.
By default, it should be set to 8 ohms.
Under the "information" section/system/ verify that it is set to 8 ohms before you start cranking it up.