A lot of headphones will have a notch around that area since that is probably an area of resonance in the ear canal. I think Etymotic once said something about this once upon a time.
In any case, I do not believe that the AKG1000 would register as completely flat under Headroom's measurement methods. They actually already probably performed tests on it, but have yet published them. Only Pre-production graphs were published which were very whacked out. However I am sure that AKG themselves according to THEIR measurement standards have produced very flat graphs with the AKG's.
Than again there is always the issue of wheter this dip is TOO much a valley, or if it is not dipped enough to which it may perhaps come off as bright?
I think a lot of people have misconceptions that brightness is in the frequency range where a lot of old guys have hearing loss as in the 15khz+ range. Brightness probably occurs much much lower...
You can compare the SOUND of the phones and say the HD600 is indeed darker perhaps because of some recesses in some higher frequencies. But I don't know if you can compare published graphs to graphs that aren't published, or graphs done by a different party. I do think Headroom should publish ALL their graphs especially for the phones they use in normalization data. And get a V6 in there as well.
That said, if I bought a product, I would value the graphs of those that USE the graphs for quality control rather than a third party. Unless you assume the company to be fraudulant in their claims or boasts. So once Headroom DOES get the Etymotic graphs working...if they come out wacky it isn't because of the phones. Also I still find...the moment you need to adapt the measurement criteria to do more accurate testing...the less the graph information is relevant to other graphs. Which is why those AKG graphs might not be as relevant to Headroom's graph whatsoever because of a lot of differing testing procedures and equipment.
Course I do agree that the headroom graphs do show the HD600 as laid-back...and to my subjective appraisal somewhat dark. And I also heard what I perceived as a slight midbass bump as headroom graphs show. And my subjective appraisal came long before any headroom graphs. But I still think most people would well enjoy the Senns more than V6. It is very hard to compare however since there is a big difference in what they are used for as well as price.
I do find it a tease that Headroom had preproduction graphs of their highest tier headphones, but no longer. And they actually USE the data in the normalization process anyhow, so without publishing those graphs, the smoothed normalize graphs mean very little to me. AKG1000, some STAX, Sony 3K, and even the Big O I believe were once visible.