When I'm
When I'm mixing or mastering, my best friend are my ears and not measurements - or at least as little as possible. Mind you, one needs to do cross referencing with reference (target) tracks and different equipment. When mixing, I rarely use headphones except for searching for errors (someone coughing, a pen falling, wrong notes, electronic buzz, etc). I agree with what you said about an imbalance of the drivers, however, I see this as a defect and not as a natural cause of a frequency affecting the imaging depth or instrument separation. Of course, If we have a problem with hearing in one of our ears, then this will also cause an imbalance. One of the first things I do when purchasing headphones or speakers, is to listen to them with different mono recordings. If there is a discrepancy between the drivers, back they go. This actually happened to me a while ago with a pair of speakers, but I was lucky enough to happen to live in the same city as the company, and was invited to go for a visit where the speakers were made in-house. I was of course given a newly matched pair.