Hi Antun
It was one of the issue I was surprised at when I was developing the BHA-1 headphone amp as to how varied the frequency response of the different headphones were. It may explain why there are so many different preferences and opinions about the listening experience. 
james
Hi James!
I see your point!
The FR graph of the Sennheiser HD800 is very flat and to my ears, they are the flattest transducer I have heard to date and this includes loudspeakers. With the BHA-1, the tonality is virtually spot-on realistic, no matter the what kind of instrument is being reproduced. Obviously, Sennheiser did it with the HD800.
Every person will have his or her opinion on the sound. Still, that doesn't mean we should change the laws of physics just to accomodate one's opinion and make it the "right" one. Equipment can either sound realistic or not, there is no other basis on which one can evaluate it. I have to tell you I searched hard and it took me 2 years to finally set on a pair headphones and then another year to find the perfect amplification for it. I always go back to my sudio recordings and live memories from the various places I played at and then I know whether it sounds as good as the real thing or it doesn't. But many, many people don't have such experiences, no point of origin, so they might get impressed by something that is farther away from the truth and in some cases, the truth isn't at all what they consider good sound.
Some manufacturers try to imprint their signature on the sound of their headphones just to stand out a bit from the crowd. That is fine of course but ultimately, it doesn't get you anywhere because someone else might say his way of doing it is the right one. The peril of this approach is compatibility. There is just no way equipment can sound good with everything if it puts it's own imprint on the sound it reproduces. Genuine neutrality is the only way to achieve this compatibility.
The BHA-1 is an utterly flat, color-free and linear-sounding amplifier. It is linear in a way that FR domain is accurate but more importantly, it is accurate at all volume levels and this has proved to be it's trait that is impossible to find elsewhere.
So, I understand your troubles but you did well and "sanity" and engineer's approach are always a good thing if you ask me.
Best,
Antun