I came from 560s and i had a very long journey. A 660s would sound muffled to you (maybe).

At the end i climbed from Hifiman XS to Hifiman Ananda to Arya. All excellent Headphones and the Arya are the first Headphones that I'm to 93,45% sure this is Endgame.

PS.: The HD700 sound amazing with Auto EQ. If you get them for ~300-400 second hand, try them. I think they are your cup of tea.