For my HD-650, I am using a LittleDot Mk V.
For my PXC-250 (also 300Ohm) [used with iPod], I am using a LittleDot Mk I.
Both are solid states.
They are both bringing all the dynamics that they high-impedence headphones loses. I have also used by Mk V as pre-amp for my Audiobytes with great (and unexpected) success.
The Mk V is about 320$ (shipped) to US and Canada while the Mk I+ (better version of Mk I) is 180$ (shipped).
They also have a lot of tube models.
I have not experimented with a lot of different headamps, but theses exceeded my expectations and I have beed very happy with them for the past 4 months and not considering anything else.
