When the Rebbe DovBer noticed that his chassidim (followers) thought that they should be devoid of any emotional appearance, he wrote a public letter stating that this is wrong.
Rabbi Shneur Zalman himself was known to pound his fist so hard on the wall while praying that he sometimes literally bled from his hand.
Rabbi Dovber, in his magnum opus Sha'ar HaYichud (The Gate of Unity), explains the entire spiritual superstructure of creation.
Rabbi Aharon, on the other hand, argued that it was dangerous to discuss certain aspects of creation because it could lead a person to inadvertently view God anthropomorphically.
Rabbi Aharon's first book, Sha'arei HaYichud ve'Ha'emuna (The Gates of Unity and Faith), is based on the section of Tanya of a similar name.
In it, Rabbi Aharon argues that the corresponding part of Tanya was incomplete, and that it is therefore necessary to learn his book in order to understand it fully.
It develops the concept of pele (wonder), which refers to the paradox caused by God's and the universe's simultaneous existence.
He argued that, contrary to Rabbi Dovber Schneuri's position, cold and intellectual contemplation cannot lead to true self-nullification.
Only by openly and emotionally desiring to cleave to God can one attain something approaching nullification before the Infinite (the highest goal of Chabad Chassidism).
While his books are not commonly studied within Chabad circles today, they are widely respected for their scholarly insights and broad scope.
Also see Louis Jacob's "Seeker of Unity: The Life And Works Of Aharon of Starosselje" (Vallentine Mitchell, 1966.)