The left driver was out of place. I moved it into place and that solved the problem... Measurements on the right driver may also be different from previous ones since that one was also re-positioned.

Frequency Response



Distortion Right



Distortion Left



CSD Right



CSD Left

Man that looks glorious, perfectly boring, nothing out of place haha.

   http://www.youtube.com/watch?v=l3P4Fv64T44

My older black driver, original pads and stock cable HD650:



and CSD (left and right superimposed)



I listen to it like this:



Note the dB scale is 5dB/div and is 'stretched' in vertical sense compared to the usual graphs shown here.
Sounds like the measurements show as well.
It's one of the headphones I will never part with.

That's impressive. What EQ filters do you use exactly?

It's an analog filter (not an parametric or fixed band equalizer) module dedicated for this specific headphone that is inserted into a portable amp.
I do not sell these amps, just design them and build them for myself.

more info can be found on my website (globe under my avatar) but as it isn't exactly pirate booty (no fancy nor any audiophile stuff in there) and still in its infancy I keep quiet about it.
Look for 'Kameleon'.

I don't use other headphones any more (except for the T50RP and DT1350 with its own modules) and stopped searching for the next (always imperfect) headphone out there.
Doesn't mean I don't enjoy trying out other headphone models (like the A5 Pro recently).

Solderdude's HD650 sounds so good that you really do not want to take it off. When he was measuring the headphones a few weeks back I was amazed by the sound. I am not a Sennheiser guy (except for the HD250 II Linear) and I was hooked.

When a headphone sounds as good as his HD650 you just enjoy the music.

The amp and filter(s) are very well documented...

It will be available in the USA in the future and is available in small quantities in Europe (as mentioned I am not selling/building them myself, just enjoying it)

And yes... you are correct all headphones are perfect... in being far from perfect ! 
As Deepfunk mentioned I only hear the music on this setup.
To me the source, amp and headphone are out of the equation, only realistic music remains (on well made recordings).

YMMV though and then there is preference as well as crappy recordings which sort-off can make some flawed headphones shine in those other occasions.

Needless to say the HD650 is an excellent headphone with very low distortion and good impulse qualities.
It just needs to be 'motivated' to perform optimally.