A sure sign that something's right.
I'd like to have another listen to the 600. I've had a couple over the years and never cared for them much, but a lot's changed since then.
I don't think I'll be changing from my 650s though.
The HD 600 and the HD 650 are both great headphones. The soundstage of the HD 650 is awesome and I find they situate me quite inside the music; the HD 600 have a narrower soundstage and make the much sound slightly further away from me.
I find the boomier bass of the HD 650 obscures some details that the HD 600 clearly reveals and for that reason listening to classical and acoustic jazz on the HD 650 is slightly frustrating, but the HD 600 sounds more evenly balanced and natural sounding, whatever type of music I play through them.
I'm grateful that I have the HD 650 and HD 600 and they're both great headphones. If I could only choose one of them, I'd opt for the HD 600.