there is no headphone with an even response. They all have dips and peaks, that said seeing as there are no perfect headphones... except maybe those Stax TOTL I hear about, but with that said everything is subjective, to some the HE 400 might be very natrual with all vocals, to my ears with a mojority of my vocal music the low mids are a little to promient for my tastes. Still all voice varry and different set ups add or remove some nuances from voices
I used to prefer my w1000x for vocals out of the M Stage, but now that it's sold, and I have the AGD and Hm801 I prefer the Dt 880 for vocals out of the AGD and the w1000x out of the HM801, it may be that with a nice tube or maybe even out of the Balanced out of the AGD the HE 400 may sound better with vocals in my opinon.
Although do appreciate you questioning every post I make :3, makes conversation interesting and allows us to think a little more about how we listen and what we are listening on!
And everything is subjective and based on preferance. I like a little color in my vocals, when I had Tubes they did a nice job enriching vocals, but often made the music kinda lack luster [cheap tubes sadly] so that said, I may enjoy the HE 400 more out of a nicer tube amp!