The sonic differences between the HD600 and HD650 are very minimal and I've never found that much difference between them. The HD600 is slightly more neutral but the HD650 has the more euphonic sounding to me.
How long have you been using them? Don’t forget the "brain burn-in".
I'm sorry but I can't agree with you at all that they are very close in comparison to each other. I have had both the 600 and 650 at the same time and the difference was major to my ears!
I was using a single power slam tube amp and did a lot of tube rolling at the time, but nothing seemed to help my 650's get out of the dark so to speak. My preference does lean more to a brighter sound, but the 650 just took out all the detail I was looking for.
I am also a Grado lover, and one of my favorite headphones at the time was my Sony SA5000's, so I think you can understand why the 650 was not good for me and again I'm sorry to people that love that sound signature.
Edited by pedtool101 - 1/25/14 at 2:16pm