The Grace is well respected on Headfi.org. If you search the threads there, you should find lots of info there.
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Check out the Apogee Mini-DAC:
You could run your 650's either from the Headphone jack in the front, or you can get a balanced cable and use the XLR outputs in the back. You can get this with or without USB (it's an add-in card) and there is also a Firewire card coming sometime.
PS Audio is also coming out with a Headphone amp in the next few months - http://www.psaudio.com. No details yet, but you can follow the discussion in their forum:
registration required for forum: http://www.psaudio.com/account/index.asp?target=%2Faccount%2Fforum%2Fview%2Easp%3FcatID%3D2%26forumID%3D5%26topicID%3D2930)
Sounds like it will have:
USB input only
RCA output (passthrough)