I have had two D600 bodies (thankfully no spotting issues - had electrical glitch in 1st body) and both were fine tuned to all of my lenses with lens align at Fort Worth Camera.
Each body had substantially different adjustments required. First one needed NO adjustment on 24-85mm VR kit lens; but second needed -8 - just as an example.
Other posts indicate sometimes a second lens adjustment test in different light is a help when the adjustment fails to yield sharp photos.
I am certain you can find in a search LOTS of info on the shooting discipline required by the D600 (& D800 & D7000 etc) and this my aid you in being sure that your 50mm is or is not focusing with this body correctly.
The D600 IMO is an excellent body, and your 50mm 1.8 is well regarded by many folks whose judgement I trust.