The HD650s have about as flat a frequency response as you can get from headphones. AKGs like the 701/2s and 712s have a greater bass extension but are nowhere near as flat. That's only part of the story in the overall sound and feel, but a large part in the colour of the cans. AKGs tend to be more mid-forward, which can help when mixing on them, as you are presented with more sound in the areas the ear is most sensitive at and picks up most detail in. But they can slightly over-emphasize an already mid-rich sound like a Marshall amp/emulator, whilst a more scooped Fender amp sound can benefit.

But in the end, you'll either like the sound or you won't.