How I compare them... Tough to describe. 900 have a slightly wider and deeper soundstage. Nothing huge though but they disappear a bit more on my head. They have more low bass and more treble extension as well and is quite a bit faster.
The 450 Pros is a bit smoother and have a bit of a more juicier sound for classical in particular. There is more flare or what to call it without sounding too coloured. They don´t have much deep bass volume which givethem a nice flow. Of course for some music you do miss that low bass too. They are more relaxed smoother and more forgiving on sibilance and such. They show it but doesn´t shout it out when watching tv series with compressed audio for example, whereas the 900 have more PRaT and even more detail.
I would say they compare quite well I go from one to the other and gets impressed pretty much always. Both is superb allrounders.