What is the difference between Shastra and the word of a pure devotee of Krishna?
It is well known that to get a clear understanding of Prabhupada’s view on a particular topic, we must look at all his statements on that topic. Prabhupada clearly taught that we follow guru-sadhu-shastra, and he made equally clear that these are three different sources of evidence. Thus, in your quote below, Prabhupada says that Bhaktivinoda Thakura’s statements are AS GOOD AS Shastra. He does not say they ARE Shastra. BVT’s and Prabhupada’s statements are as good as Shastra for a simple reason: they repeat Shastra. Prabhupada explained this hundreds of times, so I will not belabor the point.
Shastra is scripture, such as Bhagavad-gita and Srimad-bhagavatam. In his famous and Tattva-sandarbha, our definitive work on sources of authority, Srila Jiva Goswami very clearlly explains that our ultimate authority is Shastra, especially Srimad Bhagavatam, and he maintains intact the three categories of guru-sadhu-shastra, as does Krishnadasa Kaviraja, Prabhupada, and all other bona fide authorities. Prabhupada also taught that the main authority is Shastra.