Tough Stuff explicitly says it does stack.
I always read that as it only stacking with stuff like mundane armor and such - but it doesn't actually
say that, so by RAW it
does stack with Toughness powers... Interesting, and as there will be few things that negate Toughness but not the stunt*, the end result is a sort of not-quite-halfway point between Supernatural and Mythic Toughness...
*Basically - punching with some sort of silver implement (such as rings) that aren't "sharp" enough to negate Tough Stuff...