Yeah, no worries. I was kind of waiting for the Pro 900 to settle down some with more time (I've read that they're aggressive, and boomy in the bass before, about 200 hours), but will describe what differences I'm hearing...
So far the bass is h-u-g-e on the Pro 900s. Much, much more in depth and reach than on the Z1K. It's definitely somewhat lingering and boomy at this point, but it makes for a nice foundation on music with good kick drums and basslines. By contrast the bass on the Z1K is tighter and quicker, but doesn't reach anywhere near as low. Soundstage is also much wider on the Pro 900s. When I go back to the Sonys it seems much more compressed and narrow, almost claustrophobic. I'm definitely preferring the width and natural sonic picture the Pro 900s are able to create. They're both about equal when it comes to treble reach and detail. But the Ultrasone's space helps provide more space between the details giving a better sense of distinction for triangles and cymbals.
To me so far the Sonys seem to be more of what they intended-- a studio monitoring tool, with a highly accurate sound which adds nothing to what was intended in the music. The Pro 900s are more of a musical enjoyment headphone, which gets your toes tapping on just about anything. I know they have a bump in the bass, but I rather like that so far.
One other thing-- the Sonys isolate really well, while the Pro 900s don't so much with their velour pads. The pads let quite a bit of surrounding noise in. And when I take them off and seal the ear cups with my hands (as they would be when listening) I can hear the music quite well. So isolation is not one of the strong suits of the Pro 900.... but it is just enough for use at a desk at work.