(for example Sennheiser HD650 http://www.headphone.com/layout.php?top ... 0020080650
As you can see on this page it's frequency response is far from perfect (right bottom), it adds in the middle low frequencies and cuts in the highs.
But if I would put an Equaliser on my mastertrack and cut exactly as much low frequencies as this headphone adds and add some in the highs, wouldn't I get a nearly perfect frequency respone this way?