You really can't mess this soup up!
Size and sweetness of sweet potatoes vary (yes you can also use yams). So add as much or as little brown sugar/maple syrup as you like). The same goes for liquid. Add more if you want it thinner. if you feel your sweet potatoes are small, then start with only 3 cups of broth and add more later after you puree the soup.
You can also choose to add in milk, half and half or cream to make the soup more luxurious however this soup really doesn't need it..
If you don't want to toss them bread for the croutons in butter, you don't have to. Simply season them with cinnamon and sugar and bake them in a 250-degree oven until crisp.