Nothing wrong with the HD6xx. It depends on what you're trying to establish a reference FOR though.
Reference open-back? Reference warmth? Reference Neutrality? Reference sound stage? Reference Bass? Reference Detail? People create relative references based on some comparative trait. The HD650/600, while historically favored, have seen a lot of their mid-fi market being encroached on by other competitors that focused on a single trait, or several of them, and just beat the snot of them. Other headphones have become the reference for bass, soundstage, etc.
And because of the variety, not everyone has the same "starting point" that may have existed in years prior. Because of Massdrop, a lot more people started with AKG in recent years, though I'm sure the HD6xx will shake things up again. The X2s have similarly carved their place as a starting point into higher end headphones.