Originally Posted by ed45
Is it possible to measure the impedance from the output jack without opening up the amp?
Yes. You just need a (possibly DIY) cable that makes it possible to measure the output voltage, and at the same time connect a load (preferably a resistor) to the output. Use any software of your choice to generate a sine wave at a frequency where the AC voltage mode of the multimeter works reasonably well (it does not even need to be perfectly accurate, since it is only the ratio of the voltages that will matter), and measure the voltage with and without the load under otherwise identical conditions. It is easier to get accurate results if the resistance of the load is not very different (by orders of magnitude) from the output impedance, but it obviously should not be so low as to risk damaging the amplifier. Something like 33 to 100 ohms should be OK at <= 1 Vrms unloaded voltage.
Once you have measured the voltages and know the load resistance, you can use the following formula:
Zout = Zload * ((V_unloaded / V_loaded) - 1)
A somewhat more advanced topic is measuring the output impedance as a function of frequency, for example with capacitor coupled outputs. This can be done with a sound card loopback; I have a program that can generate impedance vs. frequency graphs, but a simple RMAA frequency response test is enough to show if there is significant output reactance (e.g. capacitor coupling causes a low frequency roll-off, from which the capacitance can be calculated). With simple multimeter-only testing, one could measure at a low and mid-range frequency (e.g. 50 and 400 Hz) to see if there is any serial capacitance.