From what I hear, Innerfidelity got 2 sets of the UE-4000 (from where I don't know), and had problems on the left side with both sets. Reduced bass, increased distortion. So I tested mine with deep bass and mono tracks, and listened to each earcup with the left ear (to eliminate any left-right hearing difference). The UE-4000 design allows each earcup to be placed evenly on the left ear, so differences there won't be a factor. My left and right drivers were within one decibel of each other. I don't remember buying any headphones in the past 2 years (about 30 sets total) that had significant differences between the left and right, although there was one brand and model of headphone, very low-priced, that had significant sample-to-sample differences.