Sat, just to emphasize the point, disregard the direction in the manual as to a lower impedance setting. As was said, this simply reduces the maximum voltage that the power supply section of the amplifier can output, thereby also reducing(following Ohm's Law) the maximum current and power that can be output. This is a safety-type requirement to keep the UL(Underwriters Laboratory)happy by reducing the chance of overheating, but it cuts the maximum power capacity roughly in half, and shouldn't be used. It's unclear what setting you used when you tested the 663, but in any case, keep it at, or return it to, the 8 ohm setting.
