The implication here is that headphone listening is not as worthy of better upstream equipment as speaker listening. (At least that is how I read the skeptical nature of the question, but of course it is understandable, as anyone outside the community
would think the cost or value of audiophile equipment is basically insane, but would certainly relate to the cost of a $75,000 car.)
Specifically, the HD800 is capable of achieving natural tonality of all musical objects (including vocals) that sinks deep into the pleasure centers of your brain, and does the same with spatial information allowing you to perceive the
a believable size, shape, and placement of each of these objects, adding to the enjoyment (for me at least).
And the better input you feed it, the closer to this holy grail you come. Conversely, distortion in the digital or amplification realm will be fatiguing, as in piercing highs, clipped bass, or digital artifice.
Headphones probably need more, not less, help than speakers because of the intimate relationship you have with a transducer so close to your ear.
I hear the goodness and all the characteristics of my high end tube amps and high end CD player and cables in all their glory with the HD800s, and I wouldn't have it any other way.
I love these phones, and if you love music and high end sound, the cost of upstream equipment is worth whatever you can reasonably afford.
(And of course there will be a lot of arguments about the placebo affect, etc., but I know what I hear.)
Having not had any experience with summit-fi amps/dacs I hope you can understand my skepticism. But I am skeptical in a good way just trying to gauge how good can it really get. For me its pretty good right now. Thanks for your detailed answer.