I think my answer above still applies:
http://www.head-fi.org/t/795365/sennheiser-hd800-s-impressions-thread-read-first-post-for-summary/135#post_12307437
^ This. If the music engineer likes to light up all the micro details by using a brighter headphone with slightly boosted highs, it can be a wonderful tool for deep level editing. But for finishing the tonal balance, he needs to know his gear. If he doesn't the least bit take the HD800 frequency curve into account, he'll probably tone down the highs for them to sound not too harsh/sibilant on his HD800, or severely boost the bass looking for an Audeze like impact on his HD800.
Then people listening to it with all kinds of crappy gear, which often do both taming of the highs and boosting the bass, in this case doubling the effect, won't have a very good tonal balance left, won't hear the high pitched detail any more, or will experience distortion of the overly boosted bass tones. Each HP adds its own coloration, and it should ideally do so starting from an as uncoloured as possible music file. This is what reference means to me.
Now how can you measure "uncoloured"? The best "reference" I think is orchestras, dynamically rich music with lots of different timbres, that don't go through some colouring amplification system first as pop music on concerts mostly do. Some unplugged stuff could work well too, a capella voices,... Live at the concert, you hear these in the most natural way. So there you have your reference; we try to reproduce the real world experience as faithfully as possible after all, don't we?
In the HD800 S unveiled thread, someone stated that HD800 classic is more reference, HD800 S is more coloured. I would think it otherwise...
Marketing has skewed the word reference. The more details are thrown in your face, the more you can look into the recording, so the more reference it is...
I think it's indeed helpful if not desirable to have this kind of presentation for studio work, as well as the most technically capable monitors/headphones so no details get lost. But does this still sound the same to me like when I attended that same concert? No...
The HD800 S sounds more like the real concert, the real instruments to me than the classic. It gets timbres just a tad more right. I don't hear this sibilance, details,... as pronounced in real life. I don't look through a magnifying glass all the time in real life.
So wrapping this all up: in my opinion:
HD800 S: closest to the real thing, hence most reference (or "flat" or "neutral" if you wish)
HD800 classic: MORE coloured, brighter than the real thing. Better as a studio work magnifying glass for that reason.
It's another perspective. Most people say Sennheiser added some colour, some warmth to the HD800 S. I say they took away some of the brightness of the HD800 classic