Fear of God. It seems that Reb Israel Salanter held that learning Musar was the key element that brings one to this. And Musar in his definition of the thing was a basic set of six Medieval books written by various authors. In spite of this we can see that he allowed this set to be expanded considerably. Just take a look at books quoted by his disciples as authorities in how to come to fear of God. Most are Medieval. Some were more recent. Many had wildly varying viewpoints but all held from a basic world view structure.
Incidentally there is never quoted a chasidic book by him or any of his disciples. I don't know the reason by I assume it is because they did not think that Chasidut brings to Fear of God of good character traits
From everything I have seen in chasidut I can safely say that this is true, and I doubt if you could find a chasid that would disagree with this. After all they do not think Fear of God is a worthy goal. Just ask them about it and they will start of a tirade about what is wrong with Fear of God.
But enough of the detour. I wanted to bring up the fact that the books of Musar themselves that are part of this set did not ignore the question of how to come to Fear of God. And their answer was in general not to learn Musar and it was not to learn Torah. It was to learn what was called Physics and Metaphysics. Sometimes this message is veiled as in the major work the Chovot Levavaot. Sometimes this message is unveiled as in Maimonides. But once you get used to their way of thinking you can see this in most of these books. The two exceptions are Shaari Teshuva and Mesilat Yesharim.
I do not know what they knew or would have thought about Rebbi Nachman.
I heard that the Madragat Haadam gave the books of rebbi nachman to students from chasidic background.
In the Mir yeshiva itself while the old mashgiach from Europe was still there there were most of the basic books of Breslov. Clearly there was some tradition that the books of Breslov were considered acceptable while most other books of chasidut were not. The books of Chabad were definite considered traif and non kosher in any sense.