Hard to describe really: The HD650 has more mid bass but falls off quite quickly whereas the HE 400i doesn't sound as boomy as the HD650 and it's bass goes deeper into the sub-bass region and the bass frequencies play much, much faster, so the HE400i has more impactful bass (slam). The HD650 rules the world when it comes to mids but the HE400i is no slouch either and vocals come across very well indeed. At the high end of the frequency curve, the HD650 does roll off a fair bit and the HE400i certainly has considerably more sparkle. This makes them sound more detailed and lively than the HD650 and also makes a big difference when it comes to imaging. The HD650 tends to fall into the three blob category of imaging, left, right and middle, while the HE400i lets you pinpoint each instrument in the sound stage with ease.
I can't compare to the HD700 as I've never heard it but it is probably closer to the HE400i than the HD650.
Ryan provides a nice review of the HE400i at the top of the page in his sticky thread.
In my case, my HD650 rarely get a look in since I got my HE400i about 4 months ago, I absolutely love the things.