That's where I am right now. Except that I don't honestly think, for my purposes, a superior headphone has yet been made. A true upgrade over the HD 600 for me would need to have:
1) The same exact tonal balance, including the notch at ~5 kHz (which controls sibilance and harshness) and the slight push in the upper mids--that's where the vocal magic happens
2) Better bass extension, since no matter how one tries to sugar coat it, the roll off is noticeable
3) Less bass distortion--the lowest notes on the HD 600 have obvious additional harmonic content, which, while "pleasant" sounding, makes the bass sound less precise
Even then, it would have to be offered for a reasonable price. I'm not paying four figures for it, not when the HD 600 itself cost me ~$280, and would be a fabulous buy even at the MSRP of $400. Because of the HD 600, I have zero interest in unreasonably priced flagships and ridiculous statement products that cost more than most cars and some houses.
Lately I've been concentrating on IEMs, since having a FiiO Q1 now gives me the ability to use the HD 600 (trans)portably, making closed back full sized headphones obsolete for me. Anywhere I need isolation, I would rather not have to lug a big headphone around, anyway, and anywhere I don't need isolation I can now just use my best headphone. Recently I've sampled the Etymotic MC5 (meh) and the Dunu Titan 1/FiiO EX1 (pretty impressive), but so far I haven't found anything that conjures the same magic as the HD 600. Is there a reasonably-priced (< $200) IEM that can get at least into the same zip code? Specifically, something that offers soaring vocals without that tinge of lower treble harshness a lot of IEMs seem to have.