It seems like this sour is really dependant on lemons, which might be a part of the tricky nature of the amaretto. Either way, my first try with 1:1:1 whiskey, amaretto and lemon juice turned out to be way sour. I've cut the amount of lemon juice in half, which made the drink way better.
It is quite interesting, given that someone else in the comments found the original recipe to be too sweet.