[Music Alchemist]

  Well, Audeze guys claimed that LCD-X was made for professional audio engineers.
Several famous music producers, including Ken Andrews (Nine Inch Nails, Paramore, etc) sponsored LCD-X. That's why I have included it in the list.
I'm surprised to hear that LCD-X is behind AKG K812 and HD800.

 
Most serious audiophiles I know think Audeze headphones (including the LCD-X) are pretty colored and not reference headphones at all.
 

Spoiler: LCD-X graph

 
Notice how it eventually starts going much lower than the green line, which is the flat speaker HRTF curve that neutral headphones should follow.

 

[TMRaven]

The LCD-X has a broad upper midrange and lower treble recession just like the rest of its LCD brethren.  What sets it apart is having a mid-treble spike centered around 9khz.  Tyll's graphs won't show that particular spike that well, but I definitely heard it in listening.  A lot of people will attribute the mid-treble spike to more neutrality.  I attribute to the spike as spiked treble.
 
Audeze markets it as reference grade and has people sponsoring their headphones for studio work, but it's mostly marketing on Audeze's behalf.  Before the LCD-X came along, they were showing various professionals using their LCD2 for studio purposes.
 
Now for what it's worth, The LCD-X is still a headphone that can be EQ'd to be relatively neutral.  It has a very smooth response in the bass and midrange, but upper midrange and treble area will need a more sophisticated EQ than with the HD800, which has a smoother treble region, but one that's elevated like a plateau. Whereas the HD800 could use a treble shelving (or anax mod), the X needs some boosting and reduction in various parts.  The one area LCD-X will perform better than the HD800 is low bass.  Roughly 50-60hz and below, the LCD-X's bass remains strong, while the HD800's starts to roll off.  
 
 
I haven't heard the 812 so I can't comment on that (as I said previously), but I have heard the T1.  Its treble region is far too boosted-- might as well keep the HD800.

 

[kawaivpc1]

  The LCD-X has a broad upper midrange and lower treble recession just like the rest of its LCD brethren.  What sets it apart is having a mid-treble spike centered around 9khz.  Tyll's graphs won't show that particular spike that well, but I definitely heard it in listening.  A lot of people will attribute the mid-treble spike to more neutrality.  I attribute to the spike as spiked treble.
 
Audeze markets it as reference grade and has people sponsoring their headphones for studio work, but it's mostly marketing on Audeze's behalf.  Before the LCD-X came along, they were showing various professionals using their LCD2 for studio purposes.
 
Now for what it's worth, The LCD-X is still a headphone that can be EQ'd to be relatively neutral.  It has a very smooth response in the bass and midrange, but upper midrange and treble area will need a more sophisticated EQ than with the HD800, which has a smoother treble region, but one that's elevated like a plateau. Whereas the HD800 could use a treble shelving (or anax mod), the X needs some boosting and reduction in various parts.  The one area LCD-X will perform better than the HD800 is low bass.  Roughly 50-60hz and below, the LCD-X's bass remains strong, while the HD800's starts to roll off.  
 
 
I haven't heard the 812 so I can't comment on that (as I said previously), but I have heard the T1.  Its treble region is far too boosted-- might as well keep the HD800.

 
I see... I haven't heard LCD-X. So, there's some marketing is behind this headphone. I have noticed that now they have removed all "Reference" or "Professional" words on their LCD-X website description.
https://www.audeze.com/products/headphones/lcd-x
They no longer label it as "Reference", "Neutral" or "Professional" headphone on any page of their website.
 
T1 is worse than Shure's SRH1840. I have had both and compared them directly with Geek Out 1000 and iBasso D55 DAC.
T1 didn't sound any better than SRH1840. It has difference interpretation of music but its soundstage, detail, sound signature weren't different from SRH1840 and couldn't justify 1,000 dollars price difference.
SRH1840 produces more organized sound with better arrangement representation. T1 sounded more disorganized. It places percussions and some instruments at random places. T1 is clearly made for audiophiles and music fans, not for music producers.

 

[alpha80]

   
Flattest headphone available? Not even remotely close.
 
Look at the green lines (flat speaker HRTF -- ignore the labels) in these graphs and how closely the raw measurements follow it:
http://cdn.head-fi.org/8/8d/8d765f27_STAX_SR-009SNSZ92251_Frequency_Response_HRTF.png
http://cdn.head-fi.org/a/a9/a9650bb7_Sennheiser_HD800_Frequency_Response_HRTF.png
http://cdn.head-fi.org/d/dc/dc3a7830_Audeze_LCD-2SN5325928_Frequency_Response_HRTF.png
 
Now look how far away the MDR-V6 is from being neutral:
http://www.innerfidelity.com/images/SonyMDRV6.pdf
http://www.innerfidelity.com/images/StaxSR009.pdf
http://www.innerfidelity.com/images/SennheiserHD800.pdf
http://www.innerfidelity.com/images/AudezeLCD2SN5325928.pdf
 
(I added the Audeze because its bass is perfectly neutral.)

Like I said, the graph doesn't correspond with the bass presence I hear.
Also, the most important place to be flat, isn't below 40z.  Everyone is graph crazy, and thinks flat sub bass (not bass in general) is far more important than flat mids.......and I couldn't disagree more.
 
And I've always said Audeze have great bass, but terrible upper miss and highs.  The LCD-X is the closest amongst them to neutral.  The LCD-X's successor will likely be an incredible headphone, because they show the most promise of any new headphone outfit.

 

[Music Alchemist]

  Like I said, the graph doesn't correspond with the bass presence I hear.

 
But calling it the flattest headphone available is a very big claim, and an unsubstantiated one, at that.

 

[alpha80]

   
But calling it the flattest headphone available is a very big claim, and an unsubstantiated one, at that.

I feel the claim is very well substantiated.  
In my experience, of trying several hundred of the most popular headphones we frequently discuss here on HeadFi, I would, without question, include the Sony MDR-V6 in a list of extremely neutral headphones with very flat frequency responses:
 
And this "flat frequency response" doesn't necessarily correspond with the graphs.  This is from personal experience.  Over time I've learned this.  Sometimes the lesson was expensive lol.
 
As for what i term "flat"....... The most important part of the spectrum to get flat or natural, to me and other pro audio guys, are the range from 40hz to 3kHz.  Especially from 150hz to 2kHz.  How flat a headphone is below 40z doesn't matter anywhere near as much as the instrument and vocal range to me.  I mix and produce hip hop, neo soul, reggae, and rock songs often, including some big radio stuff.   As well as remastering work for western music from the 30's to 70's.  Maybe those who listen to dubstep or EDM want some practically inaudible 10z frequency to be as prominent as a 60hz, and are willing to trade frequency response above 1kHz to do it.  But I am not.  
 
This is why I feel Planar magnetic, when used alone, is an inferior technology in the headphone world, for professional applications.  Ruler flat bass response in the nearly inaudible sub-30z range is a nice ancillary feature, but to me it is in no way an acceptable trade off for quite veiled treble and upper mids.  In no way would ruler flat sub-30z bass presence EVER coax me to buy a headphone, over one with a far more deliciously natural midrange or treble extension, which showcased all the audible, MUSICAL nuance in the vocal & instrument range of the spectrum.
 
Hence why I personally favor these headphones above all others:
Beyerdynamic T-90  
Sony MDR-V6 (I don't experience as much sub-bass roll-off as the graph indicates)
 
And have a very high opinion of these:
Beyerdynamic DT880
Shure 1840
Stax SR-009
AKG K601
AKG Q701 with bass port mod
AKG K701 with bass port mod
AKG K702 
 
Audeze comes close with the LCD-X & LCD-2, but that veiled treble is still slightly present on them.  Sennheiser and AKG come very close to perfection with the HD800 & K812, but both have slightly recessed mids to me, compared to the phones I list as the most neutral.  I think the Successors to the HD800 & K812 will remedy this remaining issue and several other small issues with those two technologically incredible headphones.  It's a shame the K812 has such an incredible design, nearly perfect bass response and quantity, and good, albeit somewhat too perfectly notched treble, for a "bright audiophile phone", but still, IMHO, is slightly recessed from 1k-5k, which makes them sound just as unnatural thru those frequencies as the HD800.

 

[alpha80]

   
I see... I haven't heard LCD-X. So, there's some marketing is behind this headphone. I have noticed that now they have removed all "Reference" or "Professional" words on their LCD-X website description.
https://www.audeze.com/products/headphones/lcd-x
They no longer label it as "Reference", "Neutral" or "Professional" headphone on any page of their website.
 
T1 is worse than Shure's SRH1840. I have had both and compared them directly with Geek Out 1000 and iBasso D55 DAC.
T1 didn't sound any better than SRH1840. It has difference interpretation of music but its soundstage, detail, sound signature weren't different from SRH1840 and couldn't justify 1,000 dollars price difference.
SRH1840 produces more organized sound with better arrangement representation. T1 sounded more disorganized. It places percussions and some instruments at random places. T1 is clearly made for audiophiles and music fans, not for music producers.

Yes, i feel the T1 is the audiophile Beyerdynamic, and the T90 is the engineer's or producers choice.  SRH1840, while not the best Shure can do (I cannot wait for it's successor), is a vastly underrated phone here on HeadFi.  I feel if it were $200, it would be as insanely good a value as the AKG K7XX for $200.

 

[kawaivpc1]

Any more opinion on this?

Can I conclude that
#1 HD800
#2 K812 PRO
#3 LCD-X
#4 TH900
#5 HD 700
#6 R70x = K712 = SRH1840
#7 HD650 = K702

is right rank for reference headphones?

 

[Music Alchemist]

Any more opinion on this?

Can I conclude that
#1 HD800
#2 K812 PRO
#3 LCD-X
#4 TH900
#5 HD 700
#6 R70x = K712 = SRH1840
#7 HD650 = K702

is right rank for reference headphones?

 
From that list, I have only owned the HD 650 and HD 700. I like the 650 a lot more than the 700. It has a darker tonal balance, but sounds more coherent to me and didn't have any major problems.

 

[TMRaven]

I would put 650 above 700 on that list-- even possibly above LCD-X, when it  comes to having a neutral frequency response.  Its bass has problems though.

 

[kawaivpc1]

I had owned HD 650 too. When I tested SRH1840, I felt HD650 was much behind in sound stage. So, I sold it few weeks later.
SRH 1840 and HD650 are similar but SRH1840 has more 3D sound.

HD800 is far more advanced than SRH1840. SRH1840 is almost primitive compared to HD800. I call it a caveman's headphone when compared to HD800.

So far, the majority of people have said HD700 is still better than HD 650... I dunno. I haven't tested it myself.

 

[Music Alchemist]

So far, the majority of people have said HD700 is still better than HD 650... I dunno. I haven't tested it myself.

 
It did a few things better, but unlike the HD 650, the HD 700 had major problems. (All merely my opinion, but if you look up reviews, you'll find that it's one of the least-liked Sennheisers overall.) I strongly doubt you could like the HD 700 more than the HD 800.

 

[kawaivpc1]

I found that Hifiman HE-6 was also intended for reference.
I added it to the list.
Let me know if you guys have any opinion about HE-6 in comparison to HD800, TH900, LCD-X, TH900.
These are all top of the line 1k headphones for pro.

 

[Music Alchemist]

I found that Hifiman HE-6 was also intended for reference.
I added it to the list.
Let me know if you guys have any opinion about HE-6 in comparison to HD800, TH900, LCD-X, TH900.
These are all top of the line 1k headphones for pro.

 
You'll want a speaker amp (or at least a very powerful headphone amp) to drive the HE-6.

 

[Danpyre]

You forgot to put the Stax SR-009 on the list in first place.  It's flatter than the HD800, and beats it in pretty much every way, except slightly losing in soundstage and imaging, but it more than makes up for it in tonality, resolution, etc.
If HD650, TH900, MDR-7506 and HD700 are reference then the HE-1000, LCD-3, LCD-2, LCD-X, etc. are reference.