Here is a diffuse field compensated graph of the HD800S's frequency response versus some other headphones with a fairly neutral-ish response, that might give you a few hints on where some EQ-ing could be useful. The HD800S is in dark purple.
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I would not try to boost the bass all the way up to match the best extended headphones above. But it looks like the 800S could maybe come up a few dB in the 40 to 70 Hz range in the bass. And it might be just a tad warm in the lower mids/upper bass around 300 Hz or so.
Treble is a little bright above 5 kHz. And it looks like the upper mids could be a tad depressed, and maybe brought a bit more forward in the 1.5 kHz range. And possibly also just a bit around 3.25 kHz. YMMV on that.
The response on this headphone is not that far from neutral to begin with, except in the bass, where it's more rolled off (like most open-back headphones). So some of the above could be a little nitpicky.
The curves are all centered in the 500-1000 Hz range on the above graph btw. And centering them to a different frequency range (such as the treble) might give you a different impression of where adjustments need to be made.