Sure. It is AC, and will only be a few volts while the signal is quite loud, so you may not be able to observe any diference between channels. However, if you feed the identical signal into both amps you can accurately measure gain difference by putting your meter from one channel's hot terminal to the other channel's hot terminal. You will probably see a few millivolts. If you also measure the amp output (either channel) you can calculate the gain difference in dB.
