They sound quite similar really... Tried both... Although yes the HD600 are less slow compared to the HD650... They are still quite slow / boring sounding to my ears.... Just my opinion... I like the HD650 and HD600.... Some of my favorite headphones and very good in a lot of ways.... Just I find them a bit slow and relaxing / boring with some genres..
Is it that you prefer a U or V shaped FR? Prefer something with more treble emphasis and pushed up bass?