I've got HD 650 too, and I think you would be wise to add Asus Xonar Essence One (which is another HDP) to your list of options. It has the option of swapping opamps, so you can tweak the sound signature to your liking. Opamps are available on ebay. Changing the operational amplifiers on E1 is fairly easy, much like putting a RAM module in your computer, and ASUS has a manual on how to do it – warranty will not be voided.
Now, I have not done this myself, I think the thing sounds great out of the box, but people who has done it are really amazed by the performance they're able to draw out of this thing. Just search the forums and see for yourself. Currently I'm waiting for a handful of opamps I bought on ebay, and I can hardly wait to see what that yields. :)