I think it could be the notorious gamma shift crap that Apple is guilty of. I've spent the day searching for a solution myself and have come up with this (haven't tried it yet, I will tomorrow.) http://bitfilms.blogspot.com/2011/08/final-cut-exports-look-different-from.html

@robbie75vr export in prores 422hq

@GregoryOfManhattan Yes I tried both original encapsulated in mov and transcoded prores422 by FCPX. They looks the same, the luma values ranges from 0 to 255.

5dtoRGB when encodes in 601 or 709 uses a colorspace with different luma min/max, I like 709, luma min/max is 16/235 and chroma is 16/240, so shadows look brighter and highlights are reduced appearing with more details.

The point is that if I simply do the same shadows/highlight correction from FCPX file (original or prores) the result is similar but with more noise and less natural color (to my eyes..).

Check out the 3 examples: