I want to hear the treble avg of 800 and lcd3. That would be interesting. 800 is not neutral. It's bright. Neither is lcd. Its a bit dark. I want to hear the baby of the two. Is it difficult to get that area flat? 5-10k?
The problem is that a headphone that had a perfectly flat frequency response would sound pretty off. thats why theres stuff like the harmon response curve, which gives a good guess at the perfect frequency response shape. But of course is still not at all perfect, or "the one". For example you want to hear a combo of the hd800 treble with the lcd 3 treble, by wanting the 5-10khz area flat. well making that area flat might not actually give you that sound at all. I'm gonna say this as purely a very observant and detail oriented person, but with no actual audio engineering experience. but the way a frequency response seems to work is that its a balancing act of sorts. to make something sound neutral you need to keep everything in balance, and making it all flat does not do that because it will make everything sound as loud as everything else, which wont really sound flat to you. you need to be able to hear the bass, the mids, and treble as distinct frequency areas, with smooth transitions, and to keep it neutral, they need to be in balance of each other. but to accomplish this, you need some dips in certain frequencies, and bumps in others. because music/sound in general does not always use every frequency between 20-20,000hz. its just a balancing act, and a consideration of the driver type/materials used, since they all need to be tuned down or up to sound good.
also on a side note, having certain frequencies peak or dip does not always correlate to changing the tone of the headphones, as in bright or dark. you can have a dark headphone with like a peaky treble, and it will still sound dark because of the overall tone the driver material gives/how its tuned in ways other than the frequency response. like hd800 drivers are a large part of why it is slightly bright and treble happy, because its frequency response does not really have any aspects of it that would make it have that overly accentuated sound signature. its mainly the ring driver, with the aluminum coated diaphragm. Compared to like fostex drivers which use the organic membrane type material and as a result have more bass, and impact, and slight organic tone (thinking of the denon d5000 which I have).
just shows how there is no real correct curve, or tuning. its just how well a company balances all the variables that are present in the headphone they are designing.