Keith, yes there's no problem with using speakers of different impedance ratings in your setup. As to the impedance setting, leave it at the higher value, despite any suggestion to the contrary in the manual. Setting it to the lower value simply reduces the maximum voltage capability, and with it the maximum current and power available. Although this reduces the chance of overheating, it also reduces the maximum performance capacity and should never be used. Your 805 has a quite powerful amplifier, including output into 4 ohms, and should have no problem in the vast majority of uses.
