Question about Behringer A800, one channel louder.

I'm running my setup using two a800s and mini dsp balanced. I bought these amps almost two years ago. was not listening to it for almost 1,5 years.

now i remembered that I found the levels on the amps to be not exact, one channel in one amp was louder. I've set it to different level and called it a day. but since i can still use warranty, is there any way to reliably check the level of channel output with multimeter? can someone help me?
 
I own a crossover from them and has the same problem. One of the balanced outputs is quieter. Using it now for knob parts.
Are You sure it isn't the input cables, the source outputs or even the speakers ? Swapped things around ? Could be just a faulty pot.
As for checking with a multimeter, put it in AC mode and play some pink noise and measure the Voltage on onset of clipping for example.
Then compute V/R Speaker = Current
Voltage * Current=Wattage.