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.
I'm glad that some people at head-fi are aware of the concept of diminishing returns... too many people here just jump towards the latest & great (& most expensive) gear when a more budget-oriented set-up can produce very similar sound w/o killing your wallet.