HD600's, to me, are still the best Sennheiser. Even though the HD800 had so many advancements, i still feel the tonality, and proportionality of the midrange, in the HD600, was the most natural Sennheiser ever produced. If only the HD800 wasn't slightly recessed from 900z to around 3k, and had better bass response, it might be the best headphone on earth. Until then, the 600 was their best offering. here's to hoping they get the HD850 right.
HD600 is what made sennheiser a household name in the pro audio business. And subsequent sales of their supposed predecessors, is mainly due to how great the HD600 was for its time, and frankly, always will be.
The HD600, is like the MDR-7506 & MDR-V6....it'll likely be in production for decades to come, and for good reason.