Your receiver is designed to be used with a powered subwoofer. If it is a powered subwoofer, your receiver does not care what impedance the actual woofer is.
If it is an unpowered subwoofer that you want to use, you need a separate amplifier to drive it. You then hook up the subwoofer output on your receiver (which is a line level output, not a speaker level output) to the input on that amplifier, and the unpowered subwoofer connects to that amplifier.
