Hi, SINAD is actually not higher with High Gain active, that graph is very easy to be misinterpreted. I've gone through the same thought process as you, but here's the answers you're looking for, follow back the discussion from here:
Recently, I have a weird problem, not sure if it's from Atom amp or not. Using it with Senn 58x, I leave Atom amp stay on all the time. Sometimes, there is sound channel bleeding (left audio is also hearable on the right, but a bit smaller on the right and vice versa). It happens randomly, but...
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So, if your headphone gets loud enough on Low Gain, then use it on Low Gain as you will have better SINAD.