For me, high end represents those products that offer a clear portal into the music. They don't obscure much if anything and they keep my foot tapping. They ought to scale well and keep me within a level of blissful deception consistently so that I won't worry that I'm not listening to live music. If I can be hoodwinked, forget about all that stuff, then I know I'm listening to high end gear. This also means that price doesn't necessarily equate to my definition of high end. Someone could gold plate a pair of iBuds and outfit them with some flawless and perfectly cut gems. These bitties would cost more than HE90's and I wouldn't consider them high end just because they have a "high end" price.
There are some wonderful giant killers out there (like the Koss KSC35's or Grado SR60's) but that max out quite early on (both of these phones don't necessarily give all that much more when provided with better power or better front ends). So, although they are incredible for the price and worthy of consideration...when looking for high end, I'd pass them on by because they don't do a lot of what other more expensive phones do.
What I want out of high end headphones:
1) detail, micro and macro resolution.
2) information presented naturally (what good is gobs of detail when it sounds like a computer generated project rather than waves flowing together to create art)
3) tonal and timbral accuracy (continuing from above, what good is wonderful flowing music if it sounds incredibly off (tinny, metallic etc. when unintended)
4) extension at both extremes
5) control at those extremes (solid impacting palpable bass, tight though decaying properly, not cutting out too short but not being overly plump either; the highs must be airy, airier the better, and they must not have sizzle or sheen
6) PRaT. I need to have the dynamics right and when that comes together we get this wonderful foot tapping experience where the groove of the music affects me emotionally. I can't stop moving, or smiling or crying etc.
If I were to think more on this I'm sure I can outline other aspects I hope the better headphones have. For now I think that is enough.
Now, are the top Grados high end? Well, let's compare with the headphones that exist in the world:
Basement: No namer brands from el cheapo to moderatley priced (50-100)
Level 2: Koss, JVC, Denon, Kenwood, Panasonic, Sanyo, Sony, some Sennheisers
Level 3: Some Sonys, some Denons, some Beyerdynamics, some AKGs, some Grados, some Senns, some Audio Technicas
Level 4: As above
Level 5 (High End): As above
This is how I generally look at the headphones market. What we regard here as quality headphones (not necessarily high end) most of the entire world would actually consider high end. That is, what is bought and sold on these boards is likely 10% of the headphones sold on the planet, maybe as much as 25% because of Sony and Sennheiser but I doubt much more. The vast majority of headphones sold are in the "no name" category or are the low end JVC, Panasonic, Sony, Sanyo types. Even the meager Koss KSC 35's offer buckets of quality compared to these offerings.
Thus, in a sense, KSC 35's at a tiny $20 or price are actually high end or...in the top 10-20% of all headphones in the world. Quite a statement!
Moving quickly up the scale regarding phones we all love and admire here, we move into the entry level scale: Koss KSC 35, Grado SR60-125's, a good number of Senns, a good number of Sonys, Beyers, AKGs, AT's etc.
The middle of the road from these phones (so now we have entered the top 10% of the world's headphones if not the top 5%) are the Grado 225-325, Senn 580, 600, Sony CD3000, Ultrazones upper phones, AKG 501 and maybe the 701, ATs...etc.
This only leaves out the very best of the best from each company, the upper percentages of the entire headphone population...RS series from Grado, GS series, Pro series, HP series, Senn 650, HE60, 90, Sony Qualia, R10, Denon 500 modified, Stax, AT 2002, WJP11, L3000, etc.
When one comments on high end...truly high end, we reached that way down on the list but when the nut cases (namely all of us) start chiming in, we narrow it down quickly to a very small subsection of the greatest phones on the planet...the top .5-1% if not less.
Of my personal criteria from above...the RS-2's, RS-1's, PS-1's, GS-1000's and HP-1000's hit on or exceed my expectations of nearly all of the 6 points if not all of them. Does that mean that a given Grado is the most resolving headphone ever for instance? No, but it doesn't have to be for me, to be considered high end. For now, perhaps only the Qualia, Etys, UE11's and maybe the K1000's could be considered as most resolving with the winner from that short list being quite subjective. Yet, what these phones do is to pull apart the notes to an extent that seems unnatural if one were to listen to a live performance. Does that mean that although they are incredibly resolving that they actually do the music justice? Perhaps...perhaps not, but it is interesting that over time, many folks prefer a more natural presentation to their music...so the resolution of the top Grados, the R10's, balanced HD650's etc...seem to fall into place. Nothing is really obscured, but rather homogenizes into a wall of music rather than quanti of sound. Could an instrument be followed if desired? Yes, but it might not jump out at a person as on something far more detailed (at first listen).
This is just a single example. A headphone doesn't have to be the best at anything or everything to be considered high end, but it ought to do a bunch of things right, among them scaling up, for me to consider them high end. So far, the top Grados provide that for me as do a myriad of other headphones of which I could be quite happy owning for life, but thus far have chosen to go with Grados as my ultimate preference.