Well, sounds like Sennheiser dropped the ball on the new 660s model.
I would suggest anyone sitting on the fence to go ahead and buy an HD600 or 650 if you are looking for a smooth, non fatiguing and relaxed listening cans. They are still available but they are being discontinued.
Review from very experienced Tyll Hertsens who lives and breathes headphones.
https://www.innerfidelity.com/conten...pen-headphones