I think a better way of looking at it is asking how much more would the HD 6x0 family cost if it were released today, rather than asking how the headphones in this family can possibly compete with modern flagships costing an order of magnitude more. Flagship headphones have gotten a lot more expensive in the past 10 years; they haven't necessarily gotten substantially better. The HD 6x0 family are Sennheiser's former flagships. They were the best the company could do when they were released (excepting the Orpheus, and a lot of Orpheus technology went into the HD 580, which was of course the progenitor of the HD 6x0). It's just that, back in 1997, hardly anybody was prepared to spend thousands of dollars on a headphone, especially not a dynamic headphone (electrostatic headphones were the premium option).
The times have obviously changed. I fully believe that if they were released today, the HD 6x0 family would be $1,000+ headphones. The only reason they cost what they do is that there's no way Sennheiser could get away with hiking up the price of something that came out almost two decades ago.