There are plenty of calculators available via Google to work out what you need to make a passive filter to emulate a speaker....
One I thought might be worth a try is at http://www.calculatoredge.com/electr...low%20pass.htm
Put in the impedance and number of stages and the calculator does the rest..... 600 ohm for line level or 8 ohm for speaker level, and more stages gives a steeper slope. May not be ideal, plenty to experiment with and worth a try...