When i take the calibration preset and i put the headphone output to external in, the sound doesnt get louder, in fact it gets a little lower, i dont see how that makes sense, shouldnt the output be like doubled if you put headphone out to ext in.
Could someone please explain this to me.
I imagine that there is some protective circuitry on the ext. in to keep silly individuals like you and me from overloading the input and frying stuff inside. 
Yeah most likely
Why bother, there’s an overload function which sounds a lot better. 
Destructive interference?
I’ve tried it when I got my first LP, it’s just a distorted sound. As I mentioned before, the filter overload sounds a lot better.
I’m not sure what happens when doing this with the calibration patch though, I guess it should be a distorted sound as well but I can’t tell for sure..