When a head-fier loaned me his pair of hd650s (older black drivers), I found that they sounded REALLY close to my pair of hd600s (silver). The biggest difference that I can hear was that the hd650 had just slightly less upper mid-range/lower treble and just a very slight bit of warmth.
Since I got my hd600s back from a friend of a few days ago, I am looking to sell them. I could also lend them out to some pirates, if they're willing to pay for the shipping.