With the mods as described in the K551 article:



I smuggled a bit and used 1/12th smoothing on this plot 
More importantly it sounds excellent !

Got another K550 in.



AKG needs to work on their quality control  .
3dB level difference in the mids and the behaviour above 3kHz differs considerably.
The differences with this one are even bigger than those of my K551 and the other K550 that was sent in.

Will have to check if the differences between L and R in the mids aren't due to leakage of the cable entry in the left cup. 

Modified !



This one came with a solid silver headphone cable (VERY microphonic and stiff)
It was open anyway to paly with the resistor values in the filter (22 Ohm in this one) so soldered (thin) copper wires to the drivers as well and routed them through the sheath and soldered a plug to it.
This way I could leave the headphone on the test rig and simply change the plug in the amp.
Results below:


This was the left stock driver, blue = silver wire, orange = copper wire,
The reason you see only one trace (the blue one) is because they measure the same.

(P.S. don't mind the Ohm scale on the right I forgot to remove it as it serves no purpose here)