[mark_in_manc]

I suppose you could measure it? Loudspeaker impedances were (more recently) sometimes quoted as the modulus of the impedance at 500Hz - some way above the mechanical resonant peak, and below the rise due to coil inductance. So if you put a resistor of a similar kind of value to that which you are expecting - say 5-20 Ohms - in series with the cabinet, and poke a 500Hz sine wave into it, and measure the voltage across the cabinet and the voltage across the resistor, you can work back to the modulus of the cabinet ip impedance.