How do you measure output impedance?

It is very simple if you can hook up a potentiometer to the output of the preamp. Use a value that has a maximum value at least 10 times the estimated output impedance, 25K ohms should be enough.

Feed the preamp a fixed level signal such as a reference 1,000 Hz tone or pink noise from a test CD.

Turn the resistance to maximum and measure the AC voltage across the pot. Then turn it down slowly until the voltage drops to 1/2 the initial value. Disconnect it from the preamp and measure the resistance, it will be equal to the output impedance of the preamp.

Contact me if you wish and I can fax some drawings if you need them.

Sorry, but you really can't simplify it any more than I described. Maybe it could be described better, but that is the process. It really isn't all that complicated. You can get the parts at Radio Shack for a few $$.

The modifications done to your preamp may have lowered or raised the output impedance depending on how it was done. If they put a resistor in series with the ouput it would raise the impedance. If they used a voltage divider it could go either way depending on the values chosen.