It seems that the ma900 has done a great exposing the smaller sound stage on the hd650 for me. After moving back to the hd650 for a few days I find myself wishing for the sound stage of the ma900. Some have said that going balanced would expand the sound stage quite a bit on the 650. I'm not willing to spend the cash on converting my system over to balanced. It will much more economical to pickup the ma900.
The question then becomes will the 650 start collecting dust? Only time will tell.