I actually listen that low. That's why it is a problem for me since I have to turn it up and lower foobar volume.
Yeah, I played with mine a little to see how the volume tracked and it is pretty much perfect all the way through low volume. There is only some imbalance at the very lowest level (7-8 o'clock) which is normal with pretty much any volume pot which isn't a stepped attenuator or something like an Alps RK-50. I wouldn't worry about it and I don't think your unit is defective.
On a side note, I'm really loving mine paired with the Ed 8 headphones! Great match!