You can actually view GoldenEars' Accudio effectiveness plots on their website - not all headphones are going to respond universally well to EQ'ing (they rank them out of 5, with 5 being the best). Their target is an averaged/weighted perceptional flat - again, there is no "true neutral." Not to get into this more (because you seem pretty resistive), but EQ is only going to act upon the frequency domain - there are still going to be differences in other domains that will influence how a headphone sounds (or sounds for you).
More information about Accudio: http://accudio.goldenears.net/Accudio/404
And their EQ Adaptability chart:
http://accudio.goldenears.net/SupportProdList
The A900X appear to score worse than the K550/551 in that respect (if you went and pulled up their measurements of the K550 and A900X, it would likely show you the before and after Accudio results, for comparison). The Soundmagic headphones you want do not appear on that list