You missed the idea. Biggest mid bass is not always what someone wants. The PRO 650 is a more flat, for an ultrasone, response which is better all around in the audiophile sense. But the sub-bass is also higher. It makes it more akin to things like what Denon does. Bigger sub bass, in my book, is better than a mid-bass hump. Try the PRO 650 if you haven't. I've gone through a few Ultrasones now and the PRO900 really is just not worth it to me, too much mid-bass presentation that takes away from the sub bass and then it's spiked treble. The PRO 650 is just a smoother listen than the PRO900 to me, and it has excellent sub bass. For less green too.
I'm a sub-bass basshead. My favorite, right now, is the Denon D5000 for closed sub-bass and otherwise near linear response for the rest of the spectrum (so that it can be used for anything, not just bass enjoyment).
Gotta try things to get an idea. Part of the hobby.
Very best,