(UTC) 
 :Headroom can arbitrarily be added at any point by adding bits to the top of your samples.  Where the inevitable problems occur is at the final playback stage, where everything is right against 0dBFS.  Even if you have calculated a perfect scaled output, you can't play it back without clipping at levels where the peaks will clip.  If you're playing it back at quieter levels, it makes more sense to always attenuate, and add your extra bits at the bottom instead.  21:53, 7 September 2015