Output Resistance Measurement Trick

Here's an easy measurement trick to find output resistance, but you need an accurate audio analyzer to use it:

Load the amp with 24 ohms and change the input voltage (sine wave) until you get exactly 24 volts out of the amp. Remove the load and subtract 24 from the new output voltage measurement. This is the output resistance in ohms! 24 ohms was picked to increase accuracy, but you can do the same thing with 8 ohms and 8 volts. The voltage measurement should have 3 digits after the decimal point for this to yield an accurate measurement.

Knowing the output resistance, you can calculate damping factor (DF = Rload/Rout). Note that output resistance can change over frequency.