I was just checking out the Denon 3805 and the specs say that it is 120w per channel at 8 ohms and 160w per channel @ 6ohms. How do you run it at 6 ohms? Does it damage the receiver or speakers running it at lower ohms?
You're receiver will work best at the 8 ohm setting. Setting the receiver to 6ohms is effectively like putting a governer on your receiver and only limits it's potential.
As for how to change the setting?? Read your manual. It will be spelled out for you in plain english.