Multiple sources. On my X3 I don't use an amp and I can balance it out. On my iPad via line out to my e17 or my BH2, I cannot do any adjustments. Same thing with my Nexus 5...no adjustments.
The weird thing is I don't notice the imbalance of my hearing on IEMs...only on headphones. My Philips L1, my M100 and my M50s...all feel like they are louder in my right ear. So maybe it's just how my ears adjust to the full sized cups that surround my ears, but I shot that theory down when I tried a Monster DNA and a M80...both of which were similar, and they are on-ears. Who knows. I'm a mess. :)
And yes, I've tried to turn the headphones around and the same issue persists...which is why I am pretty confident it's my ears.