Frankly, I don't see the new HifiMan products blowing away the already existing ones in a big way either, I have the impression that most improvements will be in the area of ergonomics, looks....but with HifiMan you never know, they've surprised before.
You can take down the treble of the HD-800 via EQ but personally -almost a philosophical point- I think that should be a last resort, the first objective should be to really get as close to the sound you want via just the headphones themselves (I'm not a big fan of coloured DACS/Amps either, same reasoning: the fewer variables in the equation, the better). Also, EQ'ing sounds easy but it's not so, there are factors that are hard to control. For instance, say you bring down 6.5 kHz by 3 dB. Fine and dandy, but those 3 dB are smeared out over an area around the frequency in question, else you 'd have too large a difference. With some EQ's you can play around with this 'attenuation distribution' factor but getting it exactly right is no mean feat.
One thing's for sure though: the HD-800, HE-6 and HE-500 are all great cans, each with their specific strong and weak points but I don't think you could go really radically, fundamentally wrong with one vs the other. It's the difference between a BMW, Mercedes and Porsche, not between a Merc and a cheap Dodge model.