Sherwood
Headphoneus Supremus
Doug Savitsky (DSavitsk on these forums) made two of the best amps for the HD800 -- the ECP DSHA- and the ECP L2. Those are unfortunately both out of production, but he's releasing another amp shortly called the Black Diamond. You can preorder that amp now, and while I haven't heard that amp with the HD800 it stands to reason that another of Doug's designs would also serve them well.