How does Audeze's "reference grade" LCD-X fit into all of this? I'm getting the feeling based on thoughts here and measurements that it's looking more like a u-shaped Audeze than a totally tonally flat Audeze. Its uncompensated FR on Innerfidelity actually look a lot like the HE-400, mins the HE-400 having higher overall treble.
Darth Obvious, it might seem a silly suggestion but why not buy second hand or borrow a few headphones in the HD600/HD650/K500/K501/K701/DT880/DT990 camp? When you have about a 1000 Dollars to spend you can try/buy one or two at the time, keep at least 500/600 Dollars in the Bank and keep the headphone you like the most in the end. When you finally find that one headphone you can sell/give back the 'lesser' headphones and your opportunity cost can be relatively low.
My apology. I misunderstood the budget you mentioned.I can only wish you the best of luck then. If you happen to find a fully working Sennheiser HD250 II for about 100 Euros, it is worth the gamble and Sennheiser still supplies spare parts.
I still say wait for 400i and 560, even after a possible early revision or two. I've had multiple experiences with the HD800, and while it was too bright for my tastes, it seemed refined and smooth enough to respond to EQ well. I'd personally EQ the HD800 to the desired neutrality then wait and see what the consensus on the new Hifman headphones are. I also wouldn't take the HD600 as an end-all-be-all headphone for neutrality, because I just can't get over its bass rolloff.