[Freo-1]

Found this link which has lots of good information about headphone response/performance.  After reviewing the information on many of the cans I've owned or listened to, it became very obvious why I am so enamored with the sound of the Stax SR 404 Signature phones.   
 
Have a look, and prepare to learn lots of good information. 
 
http://www.innerfidelity.com/headphone-data-sheet-downloads
 
 

[ajzepp]

Found this link which has lots of good information about headphone response/performance.  After reviewing the information on many of the cans I've owned or listened to, it became very obvious why I am so enamored with the sound of the Stax SR 404 Signature phones.   
 
Have a look, and prepare to learn lots of good information. 
 
http://www.innerfidelity.com/headphone-data-sheet-downloads

Great resource, but you probably don't need graphs and data to tell you that a filet tastes better than sirloin 

[adydula]

Its a good site for comparing freq graphs, impedance etc....been going here for several years.

The other site that is HUGE is headfi.com...u can get lost in the diatribe and the thousands of discussions on every conceivable type of cans, amps etc..

Many threads just for a particular set of headphones.

Also www.nwavguy.com the guy who designed the O2 and ODAC and then disappeared off the face of the planet..

His articles are very informative on what is important when it comes to amps and headphone measurements that affect the audio etc..

Its a subjective site for sure.

All the best
Alex

UPDATE: Adding this link :http://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html

It has more discussion on whats important or:

WHAT MATTERS MOST: Most of us want our gear to get out of the way so we can listen to the music as the recording engineer intended. We don’t want to listen to our headphone amp, we want to listen to the music. And if that’s the goal, the path to getting there can be summed up with one word

Accuracy - ak-yer-uh-see: “The condition or quality of being true, correct, or exact”

[Freo-1]

Well, some of us need to feed our "techno-geek" needs.   
 
There is quite a bit of information there that can perhaps help out people to make more informed choices.  Also, it may help to explain what aspects of reproduction correlate to what it is they like about the phones.   

[dB Cooper]

Another interesting headphone resource is Wikiphonia, although these Beyers shown there look a bit impractical:

[dB Cooper]

The good news is you can learn a lot about sound and headphone sound in particular. The down side is that these sites are strongly biased toward a modern sound that's often described as a 'fun' sound, meaning very deficient in one or more areas. The influence of these sites then encourages important manufacturers like Sennheiser to produce very colored headphones like the new Momentum on ear (MOE), which has a heavy, boomy upper bass and rolled off treble. This would be OK in my opinion if it were accompanied by full disclosure, but mostly it's not, and it's cloaked under the banner of 'fun' sound.

A big part of the lack of full disclosure are the freq. response curves, i.e. the basic 'signature'. If those curves were normalised in the upper (treble) part as they are in the lower (bass) portion, then we could see at a glance which ones were deficient or otherwse distorted. But the choppy curves, based on the erroneous notion that the normal human ear treble response is choppy like that, make it impractical for most users  to judge the sound at a glance.

HeadRoom and Innerfidelity both discuss the peaks and valleys in the measured high-frequency response of practically all headphones. They say it is a measurement artifact which the ear- but not the measurement setup- 'tunes out'. (If you see a curve without it, smoothing has probably been applied.) And it is what makes designing headphones to sound 'flat' at the ear difficult- engineers have to make 'educated guesses' about where subjectively 'flat' sound really is in all of that. Add to this the fact that recordings mixed on/for speakers tend to sound 'bright' on phones due to how much closer the drivers are to the ear (air is a natural 'high filter') and you've got another subjective 'call' to make as a designer. As an example, my Sennheiser 650s (which have a gentle treble rolloff on paper) sound much more natural to me than, say, AKG K701s which measure close to ruler-flat but sound bright and 'tipped-up' to me.

With all these complications taken into account, I agree completely that the goal should be sound reproduced with nothing added or subtracted- this is what I seek from any audio component.

[Noseyears]

Another interesting headphone resource is Wikiphonia, although these Beyers shown there look a bit impractical:

Wikiphonia is my top resource with vintage (Stax, Jecklin floats) and some dynamics. However, it still needs a few headphones to be added on their database.