The HD650 is much more mids-centric with overall much less bass, especially deep bass. I also have a measurement of the HD650 in my gallery.
The PRO 900 sounds much like what I remember from the old DT770 Pro, albeit a bit more extreme. The newer DT770 Consumer Edition is supposed to be more neutral.
FR graphs don't tell the whole story, but the differences between the pro 900 and HD650 in big-ban's graphs is pretty staggering.