Can I measure my tube amp's output?

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.

I guess it doesn't make much sense to measure in mono. You know it's amazing how many stinking problems I seem to have. I wanna just sit back and listen, but there is always something bugging me........grrrrrr.

Switch speaker cables at the amp end and see if the other channel is louder. Also playing your system in mono should tell if one side is louder. There are many things that can cause one channel to be or seem louder. You can measure the output from the speaker jacks but it is AC voltage.

It may not be the amp, it could be the preamp section, switch the RCA jacks not the speaker cables.. then if the channel follows louder to the other speaker, both your amps and speakers are in the clear, and you have a gain thing going on from the preamp.. In this case if your running a tube preamp its possible you have a slightly not matched pair.

Both channels measure the same. Insert Q-tip in left ear. Vigorously clean until left channel output equal to right channel output.