Those who listened to the first speech of the new Soviet leader were

B seems best to me.

Meaning: Those who listened to the first speech of the new Soviet leader were struct by its direct tone, less polemical than was common in speeches by Soviet leaders during the previous eighteen years.
The logical antecedent for its in A, B, and C is the first speech.
We can eliminate C and D for unjustified use of past perfect tense had been. The first speech made by Soviet leaders during the previous eighteen years were not background activities for the speeches made by Soviet leaders during the previous eighteen years were not background activities for the first speech of the new Soviet leader, hence there is no need to use the past perfect tense.
In addition, what is the antecedent of it in D and E? Is it tone, or first speech? We can, therefore, eliminate D and E based on pronoun ambiguity.

Between A and B, A is awkward. It is the tone of the speech that is direct. The tone of the speech is further modified as less polemical than was common in speeches by Soviet leaders during the previous eighteen years. Hence B is better than A based on the meaning of the sentence.

Those who listened to the first speech of the new Soviet leader were struck by its direct, less polemical tone than was common in speeches by Soviet leaders during the previous eighteen years.

A. its direct, less polemical tone than was common
B. its direct tone, less polemical than was common
C. its direct, less polemical tone than had been common
D. the direct tone, less polemical than it had commonly been
E. the direct tone, less polemical than it commonly was