I have a pair of Brainwavz HM5s, and I can detect a slight volume imbalance between the two cups. The cup originally on the left is a little louder for most frequencies except very high ones, and quite noticeably louder for very low ones. I know it's not the source, and since I can switch the cables between cups, I know it's not one cable or the other. The difference is slight, and in fact I didn't notice it for a couple months, but I can identify which cup is on which ear by sound alone with near 100% accuracy pretty quickly. Since there is so little difference though, I don't know whether this is to be expected even on decent headphones, or whether I should send them back.

Send them back. If you can hear it it's unacceptable.

if its audible, then its too much

Damn guys, really? Man, I have such terrible luck with headphones. OK I will send them back, thanks.