Just building a starter is not as good as acclimating the yeast. You can build a large starter, but if you pitch it into a really acidic must (with a very low pH), the acid will inhibit them because they are not acclimated to the conditions of the must.
If you rehydrate the yeast properly, and follow Hightest's approach to gradually add increasing amounts of the must to the yeast, you will build up a population of yeast that can tolerate the conditions of the must. When pitched, they will have much better chances for success.
Again, all this may be unnecessary if you find the problem is just that the pH needs to be adjusted upward.
Medsen
If you rehydrate the yeast properly, and follow Hightest's approach to gradually add increasing amounts of the must to the yeast, you will build up a population of yeast that can tolerate the conditions of the must. When pitched, they will have much better chances for success.
Again, all this may be unnecessary if you find the problem is just that the pH needs to be adjusted upward.
Medsen