I bought a pair of 650s shortly after buying my 880s. I love the 880s open, airy sound, but it gets a little thin for me, and with some songs the treble makes me wince.
The 650 is thick by comparison. Warm, and thick low-mid, like PDRM360 said. Just never felt like I got enough treble or upper-mid from them. Tried the HD600 and I felt like it was more of what I wanted: less thick with a nice upper-mid presence. To me it's more neutral and accurate than the other two.
BUT, I recently heard the 650s balanced out of Sennhesier's HDVA-600 amp and it shocked me. They sounded like the 600s with nice clarity, but had the added bonus of the signature nice, rich bass. They just sounded more impressive than the 600s. I ordered the amp and it should be here tomorrow. We'll see if my impression at home is the same. Hope so!
The perfect headphone recipe for me would be:
1.) Airy, open top of the DT-880 600 Ohm with slightly reduced treble, to reduce some of the tizziness or wince-inducing highs.
2.) Midrange clarity of the HD600
3.) Warm, thick low-mid of the HD650 (but not quite as much)
4.) Extended low frequency of the DT-880
Someone should make this!