I'd actually go one further than that and say that while there maybe more choice of manufacturers and models than when the HD600's were released, that 'sonic superiority and improvement' has actually stagnated.
Disclaimer; I'm a Sennheiser/Audeze fanboy and this is my own totally subjective opinion.
I'm in the fortunate position to be able to sample various headphones on a long-term basis and I've had exposure to some of the most expensive, highly-regarded and widely-reviewed headphones in recent times. These include models from Meze, Focal, Dan Clark, Audeze et al.
While all have been arguably of better build quality, using better materials and components I've been less than impressed with the core function of sound quality. I just don't see the value proposition in 'trading up' from my HD600, HD650, LCD2-Fazor daily drivers.
I'm not claiming that there aren't 'better' headphones than my DD's (and I was very impressed with the Meze Elites) - just that there isn't a sufficient margin of improvement over them to justify the expense of buying newer and phenomenally more expensive models.
I suspect familiarity is a major factor in that rationale however and we all know how the saying goes...
Familiarity breeds contempt.
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