How to measure pre amp for channel imbalance?

That'll work.

Get a 60 Hz test signal.

Measure the voltage from center pin to center pin.

This is the easiest/most conveninent way.  If you measure center to shield, you have to take 2 measurements. :)

hiendmmoe

With preamp not in the system

https://www.youtube.com/watch?v=PyD9cMarVJk
Feed this into an "input L&R" of you preamp, turn up the volume on your preamp to where you normally listen to.
And measure the preamp "output" one channel first (center to outside rca) with an AC volt meter set to 2v full scale, then adjust the website volume so your at say 1v or 1.5v then measure the other channels output. to see if it’s the same.

Cheers George

Hi,
i think you need a 1Khz 2V input signal, so your CD would be fine if you have a test disc. Then check the output signal of the amp at around 1V (volume) and your voltmeter set at 200V. Not very different from what @georgehifi suggests.