I've had the HD 598's for a few months and I must say I'm pretty pleased with them- they seem to have everything I want. However, (even though I don't have the money, I don't even have an amp or DAC yet) I'm still interested in an upgrade later on.
But I'm worried about the "law of diminishing returns" that is referenced so frequently here on Head-Fi. Thus, my next upgrade will probably be my last. I'm looking for a headphone with many of the qualities I value in the HD 598. (soundstage, beautiful mids, more sub-bass, little bit of sparkle, etc...)
Sennheiser HD 600- Seems to be an upgrade from my HD 598 with mostly the same sound signature. Heard it has realistic, wonderful mids, neutral presentation, comfort, plentiful detail. But, has a noticeable lack of sub-bass, a 3-blob soundstage, not very fast and rolled off treble for some.
HifiMan HE-500- From others, they're heavy, bulky, lacking timbre and requires good amping. Comes with swift speed, great soundstage, beautiful neutral presentation with realistic mids and extending bass that the Senn's don't have.
HifiMan HE-400i- Hasn't come out yet, but seems to have far better ergonomics than the prior series and tuned to be similar to both the HE-400 and HE-500.
I'm also looking for a good amp and DAC that can pair with the headphones without costing too much. In all, it should cost no more (with the headphones) than $800-$1000. If you have any experience with any of these headphones or upgrading from the HD 598, please feel free to recommend any viewpoints or new headphone suggestions.