While I'm satisfied with my new HE-400s, I have to admit that past the 200$ mark the diminishing returns really kick in. While I do think these are humongous improvement from the Sennheiser HD598s, they arent twice the headphone. I think I might have reached the absolute maximum I'm dropping into headphones.
For EQ, I ever so slightly bumped 50 - 100hz range upwards, lowered the ear piercing peak at 1,2kHz by 4db and also again at around 14kHz. Otherwise the range is quite good.