I think you are confusing preterism with idealism.
Preterist believe in actual fulfillment of prophecies that were given in symbolical terms. E.g., when Revelation speak of "the great city which spiritually is called Sodom and Egypt, where also our Lord was crucified" we take that symbol to be speaking of Jerusalem. So that later on in Revelation when we see references again to "the great city" as the harlot we know the author is again speaking of Jerusalem. These passages all predict the destruction of "the great city" which symbolically represented the actual city of ancient Jerusalem.
Further confirmation of this interpretation is given by the contrast between "the great city" in chaoters 11 and 17 with "the great city, the holy Jerusalem, descending out of heaven from God," in Rev. 21:10. It is not a coincidence that the earthly "great city" (old Jerusalem) is contrasted with the heavenly "great city" (new Jerusalem). See Galatian 4:21-31 for additional confirmation.
Ya gotta understand the symbols to understand the book. And the symbols do not exist in a vacuum as many futurists seem to think. They are interpreted by the rest of the Bible.
Does that help?
I had incorporated that....a proper gramattico-historic method would take into account any passage that has indications it should be taken symbolically/metaphorically/allegorically.
Without those CLEAR indications a passage should be taken at face value.