I forgot to add that the 650 were moded.
For those that have tight budget there is good news..
With the Jotunheim, I felt the HD650 benefited the most because the 650 sounded much more controlled on the lower spectrum and had a deep tight bass..
I would say the 650 and the HEK benefitted most in low end punch and discernable in instrument separation,
Followed by the Flow and HD800 .
The Flow was really supple in breath and bass presentation.
The fullness of bass that the Flow has on other amps,
was replaced with a really nice supple controlled bass breath you could sense, which also the HEK had.
The 650 also conveyed nice controlled bass extension which was really SO impressive it made my HD800 anemic .
Usually I do not like the 650 because it always came across in the past as a dark, thick , or a bit dull headphone to me.
But thru the Jotunheim, it came across as very natural, almost neutral(!)
My Second fave of the bunch!
As for the HD800
The increased treble presence and lighter, quicker bass made it less neutral, and less powerful sounding than the others,
And made the HD800 the last choice here...
Not because it was stock, as my HD800 has ALL the mods and more than any other I know or ever read, but I still not completely happy with it.
The 800 is an never ending work in progress that in the end needs amp matching.
So it was still the last pick here..
The point is that the Jotunheim has great authority over all these headphones and makes all of them climb higher than you would imagine.
I'm not saying it is the best amp,
But it has the clarity, power, and control of a high end amp.