WOW - where to start. I may need a little help from my road dog = Happy Camper..
While I have not heard the Dark Star, someone has who's ears I trust. It was said that a B24 (DIY speaker amp) "Killed" the Dark Star in a side by side with the HE-6.
I've had the EF-6 in my home for a few months (now gone). This was the best "headphone" amp I've heard with the HE-6, the next best "headphone amp with the HE-6 to my ears would be the GS-X mk2. I don't know for sure, but I've been told the Dark Star is nothing more than a headphone amp with very high gain.
The following headphone amps was a "No go" for me with the HE-6.
Burson HA - 160D
4 channel B22 = the third best headphone amp
The only way to satisfy this question is to go on a journey and try as many headphone amps and speaker amps as you can.
The only consensus is, there will never be a consensus:D But really, I've always tried to stay with Pure Class A amps. Why? I have no idea.. I just think it provides the cleanest power there is.
TBH - I'm not much into the synergy thing opposed to the quality of the amp for the HE-6. Yes, I've found the more expensive the speaker amps gets - The better the HE-6 sound. This may not be the case for others.
If I could afford this amp - I would have it right now - this would be my END GAME HE-6 amp. http://www.simaudio.com/moon600i.htm $8K I believe.
I've mentioned before. The Emotiva a-100 mini-X is a great starter speaker amp for the HE-6. This amp sounds better than most headphone amps on the HE-6... Most, not all. There are also other smallish speaker amps in this price range that sound pretty good as well.
I think the key here is "high quality" The HE-6 is picky towards quality. Just like the Emotiva a-100 mini-X. This is not a high quality amp - it has it's flaws and the HE-6 will show them. One being the brightness another being shabby bass. These things and others will stick out like a sore thumb on lower quality amps.
So I'm not sure if the HE-6 is picky towards a amps sound sig as much as it's picky towards the quality of amplification you feed it.
IMO The HE-6 is "only" on the bright side based on the quality of amp you feed it.. Again, I don't get any kind of brightness at all with my F1J. Now the T70s are bright, you have to put in some work to get the HD800s not to sound bright, Some Grados are bright. If the HE-6 is bright and etched IMO its a tell tale that the amp your using is not a good amp is some areas, like taming the treble.
I went out and got a "true to the sound" amp. The F1J is one of the cleanest sounding, most revealing, dynamic and transparent amps in the $1500 - $2000 range - that I've heard. Much like the GS-X mk2 but a few levels up - IMO.
With anything, it get's to a point of diminishing returns and to a point where the differences are very small if any. So I would guess with a 8K amp and a 10K amp that's meant to drive speakers and not headphones the differences would not even be audible.
One quibble I might have is that I am not sure that the HE-6 is so much picky about.qualty as if is revealing. It is not bad with lesser stuff, but once you hear it with a real good set up (or two) you realize what it can do. It is like having good speakers which let you hear how good your other components are. Definitely audition.