It's kind of sad that nobody knows, considering how popular this is.
You're right that it should be fairly easy to measure, though how accurately is going to depend on your gear. With just a couple of 1% resistors of different values (just make sure they can dissipate that much power, though that shouldn't be a stretch at lower voltages) and a cheap multimeter, you should be able to get a reasonable estimate with a couple simple measurements and calculations. Just set up the source into the load resistor for a simple voltage divider.
V_L = V_s * Z_L / (Z_L + Z_s)
V_L = voltage across load resistor, measured by multimeter
V_s = source voltage (playing back the same test tone, should be the same for multiple tests), probably unknown
Z_L = load impedance of the resistor you are using, can be measured too (and should be close to nominal value with a 1% tolerance resistor)
Z_s = source output impedance, unknown
With at least two resistor values and V_L measurements you should be good to go. 2 (or more) equations and 2 unknowns.
Make sure to report back what you find!