Chuck,
If you are unsure of the output impedance, one solution is to add a low noise buffer (op amp), with a 50 ohm series output resistor and large DC blocking capacitor (passing 600 Hz would require about 22 uF to minimize impact on amplitude accuracy). You could also add some gain here depending on the expected signal level.
If this is not an option, you could try measuring the output impedance, or alternately see how much the amplitude changes when you add a e.g. 100 ohm series resistor to your output, and then use standard resistor divider equations to solve for output impedance (assuming a 50 ohm input impedance).
