Amer,there isn't an 8/4ohm switch on the back, but I know that the 797(which Chess has)and I'd presume the 787, both have a sub-menu during setup which allows selecting a 4ohm setting. Check your manual and try it. Since the 4ohm setting limits the voltage and therefore the current flow and available power, it should only be used when necessary, although owners manuals, in an excess of caution, usually say to use it whenever 4ohm speakers are present. Let us know what you find.

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Enjoy the music, not the equipment.