I'll start by saying that, because the circuits and tubes are similar, the sonics are similar. The differences are more like different seasoning than completely different recipes.
The Green Crack has more midrange emphasis, more of the tube sound that some users might want to tame aggressive or less refined digital sources. It might also be a good choice for a headphone that has a somewhat scooped frequency response or is a little bass light.
The C2A's shunt regulation give it the edge in resolution, refinement and frequency extension at both ends of the range. It might be the better choice with a more refined sounding source. The response is flatter sounding and I found one of the biggest differences to be that the bass was cleaner. Some might infer the bass to be lighter, I found it to be more even, quicker and to go deeper. I also heard comments to the effect of better imaging. I can't offer up much in that regard as I don't find headphones to image well enough for it to be an issue. The comments may have been related to the top end being a little cleaner and thus giving the impression of more air.
These impressions are based upon listening with Anax modded HD 800s. YMMV.
