Haven't heard the Graham Slee and don't own the HD650 so I can't comment on those two. It's been around two years since I've had the CD5004 and have used it with D2000, HE-500, LCD-2 and LCD-3 in the meantime. Still a great DAC and pushes for transparency. It sounds more on the analog sound rather than cold digital sounding. It's quite neutral, nothing exaggerated. Good soundstage for sure. It should work with HD650s and give them good body provided that the amp delivers.
Thanks for the quick response, glad to hear the positive comments. By the sound of it, it will compliment my setup really well. Think I will take the plunge, can't really go wrong for £158.