You will only get the DC value but for a DAC, the impedance is probably relatively flat up to at least 10kHz. It won't be quite right of course, but it will give you an indication of the overall impedance.
IF it is a tube circuit that uses a cathode follwer as an output buffer then you could measure the resistance as Aball did with the power off and be very close, but this is not the way it is usually done.
1. Play a steady test tone from a CD and measure the AC output voltage with no load.
2. Hook up a potentiometer as the load to the dac. It's maximum value should be greater than the output impedance of the DAC. A 10K Radio Shack pot should work fine.
3. Play the same test tone and reduce the value of the pot until you measure 1/2 of the AC voltage you had with no load.
4. Unhook the pot from the circuit and measure the resistance. This will be equal to the output impedance of the DAC at the frequency of the test tone.