Yesterday I bought a cheap HD-650. Have owned my HD600 for 2 years now, but before that I actually had the HD650.
Now I wanted to try them out again, but they are nowhere good as the HD600
I dont even think they have better bass. They have slightly more bass at the 80-120hz region, giving them a more punch, but in the same time a much softer punch, where the HD600 sound more solid and distinct.
For soundstage HD650 have a slightly larger one. Or wider I should mention. But then again soundstage is more 3-BLOB. Vocals infront and the rest at right and left.
The HD600s soundstage feels more circular and definatly more airy - which I prefer.
The only thing HD650 really does better though is that less grainy sound it have.
Now I will sell these again :)