A pinch of baking soda.

Sugar, wine, or a little cream should do the trick. Also, sometimes canned tomatoes just need to simmer for a while to get their natural sugars in action. Try simmering for a while (add some extra broth or water), and then try adding another ingredient.

Try some "sweet" veggies. . . .carrots always help take the acidic edge off tomatoes.

I would go with the cream (or milk) answer but in proportion to the soup. Don't overwhelm it. Depending on where you live you might be able to buy Mexican crema cf. creme fraice, sour cream etc. That would work too.

Sorry, that should have been "isn't enough." Really must read my answers before submitting . . . . ;o)

A tiny pinch of sugar should do the trick. And if one pinch isn't even, add a second, but taste carefully after each addition, as it is easy to overdo it. A touch of cream also will help smooth it out nicely.