Those two are totally different tuning wise while the other HD 600 series aren't exactly. Though, I don't think the tuning choices they made for HD 660 S make much sense. At least I don't get it. DT1990 and Hifiman Sundara have recessed upper mids and strong treble peak around 9K while HD 660 S is flat in comparison (more upper mids). Other HD 600 series cans have even more upper mids and have very smooth frequency response without large dips or peaks.