Answer: No. The Preface to the Wisdom of Kabbalah and The Study of the Ten Sefirot are auxiliary books for the development of the soul in comparision wiht the articles and letters of Baal HaSulam and Rabash. None of the sources has greater influence on the soul than the articles of Shamati.
Question: What is special about the letters of Baal HaSulam compared to Shamati?
Answer: Baal HaSulam raises a certain issue in every article in Shamati and explains very specifically and clearly what it is about.
In the letters, on the other hand, he writes about daily matters, for example an article about Parashat HaShavua (the weekly Torah portion) and things like that. Generally speaking it is not a methodological presentation and expression of the material. But the truth is that there are parts in it that we simply swallow and cannot leave.
From KabTV’s “Fundamentals of Kabbalah” 3/10/19