It depends on your budget. Of course the Bottlehead Crack is a favorite among a lot of HD650 owners, but of course you do have to build it. But also, you may be able to find one in the Head-fi classifieds that is already built or ask Bottlehead to build it for you (for an extra fee). I am really liking the Schiit Vali with the HD650's, it pairs well with them for me, however it depends if you like tubes or solid state. Other options are the Schiit Valhalla, Asgard 2, Magni, or Schiit Lyr. Yes i only named 2 brands the Crack and Schitt, but those seem to be the most popular amongst HD650 owners.
It really comes down to your budget, whether or not you want tube or solid state and the sound signature you are looking for.
Edited by TheGame - 12/7/13 at 9:52am