You keep mentioning channel imbalance. Unless there is something wrong electrically somewhere there should be none. It is very easy however to get imbalance, all it takes is one speaker one inch closer than the other, either to you or a wall or something that might reinforce, or some furniture or something that might absorb. Or even if everything else is right but one is toed slightly more or less than the other. You get the idea. Everything has to be perfectly symmetrical down to the nth degree. Which is why I always say start with the tape measure.
If you are gonna measure anything, measure distance. Measure angles. Those you can get a handle on. Try and measure sound, you are gonna find you got your work cut out for you just trying to figure out what it is you are really measuring. Then why. Finally how. Guarantee in the end wind up learning, nobody agrees on even the tiniest aspect of any of it. Where hopefully everyone agrees if the speakers are cockeyed so is the sound.