Why do you juxtapose the two?
I don't see any contrast between the traditional Reformed practice of redemptive-historical preaching (which is nothing more than tracing out the progressive revelation of Christ in Scripture) and the perspicuity of Scripture. I just wrote a piece for Mod-Ref that will appear in a few months on this very topic.
As for the perspicuity of Scripture:
Luke 24, 2 Cor 1, 1 Cor 10, John 5-8, 1 Pet 1, and all of the book of Hebrews all read the Hebrew Scriptures redemptive-historically. They all see God the Son revealed throughout Scripture and finally revealed in the last days (HC 19) in the Father's well-beloved Son. These passages do not treat the progressive revelation of God the Son as a gnostic secret. It's true that some folks do make a practice of pulling a rabbit out of a hat, but that's not essential to good redemptive-historical preaching. Read Ed Clowney. Read Vos' chapel talks. Read Poythress book on Christ in Moses.
rsc