I recently went from the HD595 to the HD650 and am VERY happy with the upgrade. I think that the overall sound signature is still quite similar, but the HD650s have a lot more balls (for lack of a better word). But they definitely need an amp to realize the advantages, I'm currently using the Schiit Valhalla which I like quite a bit.
That said, I think the HD595 are still a much better value (it's my go-to recommendation for friends / family looking for "good" headphones but who would look at me like an insane person for recommending $450-500 headphones). There's no way the 2.5x price buys you 2.5x the performance, but that's nothing new in the audio world. I still think the gap between the two is pretty significant and it was money very well spent.
