What makes something high end? Price? People say these compare favourably to the TH-900 which is 3x the price. Certainly when I owned the HD800 there was nothing there for me, would take the TH-X00 in a heart beat.
These cans are really glorious. Look, feel, sound... ticks all the boxes for me. Set and forget comfort, they fit like a glove for me. There are a few genres I probably wouldn't use them for but for rock and metal especially they're fantastic.
Knowing what system you heard the HD800 on would help to gauge - I indeed think they are high end.
The TH-X00 gives, to my ears, a very approachable sound. And in that sense you could consider it to be high end. But the best sound heard from the TH-X00 does not, to my ears, match the best sound heard from the HD800. There's just more depth, more palpability to the sound through the HD800. That's what I consider to be high end.
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I do not care for the Fostex TH-900. I consider the Denon AH-D7000 to be more high end.