Yeah I wont even bother testing with that. If I can't detect imbalance issues during practical usage, then I don't know why I'd even begin to worry myself over such things as that.
For what it's worth, I've followed the HE-400 threads closely since they first came out. Other than the very first rev of the he-400 where one driver would go out or have problems over time, I don't remember anybody else reporting imbalance issues on their pair.
I certainly don't want reports such as that to sway anybody against a purchase, because it obviously drove enough of a stir to get even Justin of Headamp to question it a little.
All FR measurements of HE-400 I've seen have shown have shown the response of both drivers to be relatively even with each other, I don't see any legitimate cause for concern here like the cases we had with the DT1350, Pro900 etc.