Just leave it on 8 ohms. In my main room I have a mix of 8 ohm energy towers and 6 ohm Jamo towers. All powered by my Denon receiver. Never had any problem and the receiver does not have a switch.
I have owned 5 different receivers from Denon, Harman Kardon and Marantz over the past few years and none of them had a 6/8 ohm switch. Why is this even necessary?
Is the power supply that bad in the Pioneer 1019 that one needs to change the setting?