Let me second your thought for the C390DD. I havent heard it, but from what I've read I wouldn't have any reservations sound quality wise.
The reason I'd opt for the NAD (or Peachtree Grand Integrated, the only other amp/dac to take this approach, albeit for more money) is that it has a high-pass function (40-200hz). Unless you only listen to girls with guitars in your bedroom, the very best thing you could do for the Kefs would be to relieve them of low and even mid bass. The Kef appears to be a world class speaker in everything but bass - expecting a 5" driver to do it all is silly. Build yourself a pair of Rhythmik servo subs on the cheap and cross them over to the LS50 at 150hz and you've probably got something approaching the quality of the Blades for a fraction of a price.