What is the correct consistency of a cream soup?
The 'correct' consistency is very opinionable. The 'coat the back of a spoon' instruction means that you dip a spoon in the cream, then run your finger over the back of the spoon, and if the soup stays apart, it's at least as thick as it should be... but maybe thicker (than you'd like).

The problem is the amount of time that the soup stays on the stove, as it keeps on thickening as moisture evaporates. That is why you'll get heavy soups. Add water, to make the soup as thin as your own preference.