I think your question probably belongs on the D600 Forum. I don't think you're describing a lens issue. The only ways that I can think of to cause both the viewfinder and live view to appear dim are to either activate dof (depth of field) preview or attach a filter that would cut a lot of light (e.g., a neutral density filter or maybe a polarizer).
I think the folks on that forum might have some better thoughts. Good luck.