I got to vote Bottlehead too as i just built a S.E.X amp as i wanted a valve amp that could drive low impedance cans. The other candidate was a Woo WA6 but the S.E.X. was better value for me.
With the other $5-600 you have a pick of alot of DAC's. Though you cant go wrong with an ODAC or Modi and buy a nice pair of headphones with the change..