I've known it for a while, but after listening to planars and solid state amps for about a year I've dug out my HD650s again. With the SS amp there was something not quite right after the planars, but now I've swapped in one of my Eufonika OTL amps, they are sounding great and I remember why I liked them so much.
Just saying, for anyone who hasn't got on with the HD650s in the past, but never tried then with valves, do give them another chance, it's worth it.
I have yet to try an otl amp with my hd600s, you do see a lot of positive comments about the synergy between the two.
I used a Burson Soloist SL MK2 for a while to good effect with the 600s, easily driving them on low gain setting, it is a powerful amp.
But I tried a Beresford Caiman SEG dac and found I really liked the pairing with the 600s, particularly the extended bass. I run the DAC off a 12v lead acid battery.
I also have a pair of hd660s headphones, they are capable of greater resolution and detail than the 600s and I get amazing results using them with a little fiio dap, but somehow I still prefer the Beresford - hd600 combo.
I did try a Icon Audio HP8 valve headphone amplifier (transformer coupled) it stayed with me for one day and went straight back to the dealer, I found the background mains hum was completely unacceptable, not sure if that particular unit was atypical or if they are all like that.
I should say I use icon audio amplifiers in my main speaker system and absolutely love them.