My thought is that if you have such an extensive list of headphones you lust for, you really should go to a meet/show where you will have a chance to try many of them. Clearly, you've got a lot of decisions to make (closed/open/IEM, dark/neutral/bright/etc., ethereal/palpable, revealing/masking....)
That said, as an old-time Head-Fi metalhead, I respect your dilemma, and appreciate your enthusiam, so I will give you a brief primer on my thoughts on this topic, developed over the course of well over a decade worth of high-end headphone experience.
What I look for in headphones for metal listening:
1) Transparency to the source/recording: This is paramount, as most metal is a rather dense, layered sound, and a more transparent headphone will offer a great ability to hear into that sonic jungle. This primarily pertains to how quickly a headphone responds to the incoming signal to create a deep, nuanced, representation thereof, but to a more minor extent the frequency response as well, where major fluctuations from neutral distract from transparency. Typically, in my experience, a more transparent headphone will tend to produce a sound that feels less "inside the head" than a less transparent headphone, and the more transparent headphone will build sonic "images" that are more precise/pinpoint.
2) Physical/palpable bass presentation: This implies bass that can be felt, in addition to being heard, which by and large rules out electrostatic headphones (as, by nature of their design, they do not move the large quantities of air required to feel bass via bone conduction). The quicker, more responsive the driver, the better controlled this bass presentation can be, and the more realistic the sound as a result. Planar headphones as a group excel here, combining tactile bass response with superior control as compared to most dynamic headphones. This is also an area where IEM's simply due to the nature of their application, also struggle; however, given their unique properties regarding portability and isolation, I would typically consider them somewhat apart from traditional headphones anyway.
3) Seasoning to taste. This is where personal preference really comes in, as well as fit to particular genres or recordings. One could find a single headphone that suits their collection "well enough" or try to cover different material with the sounds of different headphones, deciding which to use when.
As such, based on the more objective criteria 1 and 2, my preference for metal listening in this day and age is for planar magnetic headphones, when affordable, as I have simply found they offer a better combination of transparency and bass performance than do dynamic or electrostatic headphones, the Sennheiser HD800 being an exception to that rule (though it has its' own, non-standard dynamic driver anyway).
Here's a group of headphones that I feel all do very well in #1 and #2 for their prices (have personal experience with all), are sub $1k (if barely in some cases), in current production (or in stock at very least in the case of two) and do not require $1K+ amplification to sound close to their potential:
Group 1
Audeze EL-8
Fostex TH-600
Hifiman HE-560, HE-500, HE-400i, HE-400
LFF/Engimatic Paradox and Paradox Slant
MrSpeakers Mad Dog, Alpha Dog, and Alpha Prime
Oppo PM-2
I would take any of the above headphones without question over the standard dyanmic driver headphones that proliferate in the $200-600 range, such as the HD580/600/650, DT770/880/990, RS-1/2, ATH-A/AD/Wxxxx. Choice of which headphone would get into criteria 3, as well as fit/comfort, which is where there is no substitute for trying in person.
You can certainly get a higher level of overall performance going higher up the chain, but that often comes at the expense of more severe amplification needs as well. I've listed the HE-560 here as well, because I feel that it does scale up with better amplification, and is overall a match for the HE-6 when well-powered, though the two have differing presentations and strengths/weaknesses. *I have listed the Stax for their level of transparency, despite their non-tactile electrostatic bass presentation (which would not necessarily be a problem for all listeners).
Group 2
Audeze LCD-3f, LCD-X
Hifiman HE-560, HE-6
MrSpeakers Ether
Sennheiser HD800
(Stax SR-007mkII, SR507)*
Again, the same caveats apply regarding fit and sonic taste.
Based on several listening sessions I have had with them over the course of the year, I do feel that the objective performance of the Hifiman HE-1000 is on another level from that tier of headphones, as they produce a level of transparency comparable to the best electrostatic headphones, while also managing to provide excellent palpable bass response, for the best overall performance in criteria 1 and 2 that I have personally heard. *I have listed the Stax for their level of transparency, despite their non-tactile electrostatic bass presentation (which would not necessarily be a problem for all listeners).
Group 3
Hifiman HE-1000
(Stax SR-009)*
These recommendations are regarding headphones in current production, so no discontinued unicorns on this list. I did not mention the JPS Labs Abyss, as I have not gotten to listen to it extensively with known supporting equipment, though my one experience with it was somewhat sonically and physically trying, I would like to hear it again under more controlled circumstances.
Take that as a rough guideline, and get out there and listen to some headphones. No amount of talk here is ever substitute for trying them yourselves!
One more point regarding unobtanium discontinued unicorn headphones: do not be swayed by their market value into believing that they perform better than they really do. Most of them were loved in their time due to the lack of the amazing selection of top headphones now available. While many have redeeming characteristics, they also have their warts as well. Do yourself a favor, and unless you are a collector, don't be distracted by them! There are headphones out there now every bit equal to, if not better than the Sony R10, Sennheiser HE90, Audio-Technica L3000, and all those other headphones you've read about or seen pictures of, but can't get. Focus on what you can get, and don't overpay for what you can't.
