LumaSharpen is already in gamma space, so if you do the linear to gamma space conversion on it you end up doing gamma twice which makes it really dark.
So you either need to do a gamma to linear after Lumasharpen , then do SMAA which works in Linear space and then do linear to gamma after again.
Or do lumasharpen in Linear space which means using the linear sampler instead of the gamma sampler - doing it in linear space will affect how the sharpening will look - I'm not sure if it will be better or worse though.
If I could I'd like to do all the samples in linear space because that is faster on nvidia hardware.
Anyways as I said .. not so simple.