This is Piotr's own provided measurement of the 8.2:
And this is Rin Choi's measurement of the NT6, albeit compensated and smoothed:
However these are two more random measurements of the NT6:
I'm growing tired of repeating myself. The reason is not due to a 1.5 KHz peak. At all. Interpreting frequencies is understanding that the specific points within a frequency range are only one part of the sound. Implementing a 7 KHz peak for instance will not always mean a sound is bright. A 2-3 KHz bump does not automatically mean vocals will sound natural. And a 1.5 KHz bump certainly does not mean that vocals will sound off. Properly interpreting a frequency response involves understanding both the effects of specific frequencies, as well the balance and coherency of the frequency response as a whole.
So no, the main reason is not that peak. It is the large discrepancy between the peak and the dip what causes the effect. If the peak was at 3 KHz followed by a similar dip, the tuning would still be off, although it would manifest itself in a different way.