I don't currently have one on me. How would I directly connect the meter to the driver?
Not sure how it would help though. The drivers are well matched other than those imbalances that are on both sides, not just one.
I could see the impedance possibly being mismatched if all the imbalance was on one side, but they aren't. As Jerg stated, its more than likely FR variation which isn't uncommon.
This is probably a normal occurence with the HE400, but since people dont go out of their way to test frequency imbalances with a constant tone and sweep, they would never notice it in normal uses. I sure didn't. Makes me wish I never used sinegen to test my headphones.