They are different headphones. I think the HD598 is a bit rough and brittle up top. It is also on the brighter side so if your sources are not running the 650 good the HD598 may appear cleaner, more open. 650 has a darker background it's very full sounding and a lot of people are not used to fullness.
The 650 is more accurate but the thing with accuracy, is it is not always exciting. Most people like a more coloured, exciting sound and there's nothing wrong with that but if you want a more complete sound, you'd struggle to find it with an open dynamic headphone and the 650 does very well.
The 650 on my Mojo alone, sounded on par with the LCD-2 I had and bested it in resolution and tonal balance. On a better amp, the 650 would have beat it in every area apart from sub bass extension.