Perhaps one of the things that remains difficult about the whole doctrine of Sola Scriptura is the notorious ambiguity concerning its definition. To be perfectly honest, I'm not entirely sure where Sola Scriptura stops and the three-pronged emphasis on "Scripture, Tradition, Reason" starts. I think the least I could say is that the conception of Sola Scriptura where the Bible is interpreted in a vacuum simply does not exist in any kind of historical context. If one goes to the famous Diet of Worms, Martin Luther is recorded as saying:
Unless I am convicted by the testimony of Scripture or by evident reason - for I trust neither in popes nor in councils alone, since it is obvious that they have often erred and contradicted themselves - I am convicted by the Scripture which I have mentioned and my conscience is captive by the Word of God.
This certainly places Scripture in a high position, but what kind of Sola Scriptura is it really? Certainly Luther seems to have rejected the infallibility of tradition, but he clearly sees reason as an important element in the interpretive process. Importantly, Luther sees that a correct understanding of Scripture is not always self-evident and thus that the interpretation of the Scriptures does not occur in a vacuum. Indeed, shortly thereafter Luther and Melanchthon are responsible for the creation of the Augsburg Confession, a statement that extensively quotes the Church Fathers. I can't imagine any reason for doing this if Luther did not see the importance and authority of tradition.
Perhaps the question I'd like to know is whether Scripture is truly the definitive authority when tradition and reason are used in the interpretive process? Of course, the common appeal is that tradition and reason are merely aids to understanding the authoritative text and so in this way it is said that the doctrine of Sola Scriptura is preserved. But once any text (for example, Calvin's "Institutes") is understood to be a reliable exposition of Scripture, doesn't this become the authority in the place of the Bible? And perhaps even beyond this, is it not the individual making the determination about what tradition correctly understands Scripture and what type of reason (think different approaches to systematic theology) achieves this purpose? What then can be said to the Catholic accusation that Protestants have merely replaced the authority of the Church and the magisterium with the convictions of the individual? That is quite literally, that each and every Protestant is a pope of his or her own? I would tend to suggest that some 30,000 protestant denomination attest to this fact, rather than to the proposition that it is Scripture alone that is regarded as authoritative in the Protestant worldview.