Just want to add my bit of love for the HD650's.
Sporting my pair now, through a Little Dot Mk III with stock tubes.
I've never heard anything like it before, but like others have said, they're a bitch for upgrades.
All I keep thinking about now is which DAC to buy, which portable headphone amp to buy, when I can justify spending money on upgrading perfectly sufficient stock headphone cables, when to buy more tubes for the Little Dot, when to buy a new amp (and I've only had this one a few weeks!).
Ahh yeah, truly, they're wonderful headphones though.