Apple TV comes to mind, but won't output via USB, you'd have to use optical.
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I would suggest a Macbook or Mac Mini running Pure Music. Your NAS is connected to the Mac via ethernet, and can be in another part of the house if desired (mine's in the basement). Your files stay on your NAS but you create a library in iTunes. You control using the Remote app from the iTunes store.
This works like a charm and is the best solution (IMHO) other than a dedicated server.
Also, I've tried just about every Mac based playback software available (Amarra, Pure Music, Audirvana, Decibel, Bitperfect, Fidelia), and I always came back to Pure Music as the best for both sound and functionality.
Hope this helps.