Hmm, I've pretty much had the opposite experience. Granted I've only listened to the HD650, but unless the HD600's soundstage is much better than that of the HD650 I felt the HE-400i's soundstage was not only wider, but also imaging was much better (mainly because everything in the HD650 was so close together and thus separation suffered).
Agree with you that the sound stage of the HE-400i is wider than the HD-650 but I never heard a HD-600 so can't say for sure on my part.