How is Jesus God's final word? (Protestant Perspective)

Much of what I said here in relation to the Catholic perspective on this issue remains directly relevant to a Protestant perspective. The incarnation provides the key to understanding why 'the Son' constitutes superior revelation to all preceding and subsequent prophets - in Jesus own words:

If you had known me, you would have known my Father also. From now on you do know him and have seen him. - John 14:7 ESV

...Whoever has seen me has seen the Father... - John 14:9b ESV

Also highly relevant are Colossians 1:11-20 -

11 May you be strengthened with all power, according to his glorious might, for all endurance and patience with joy, 12 giving thanks to the Father, who has qualified you to share in the inheritance of the saints in light. 13 He has delivered us from the domain of darkness and transferred us to the kingdom of his beloved Son, 14 in whom we have redemption, the forgiveness of sins. 15 He is the image of the invisible God, the firstborn of all creation. 16 For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. 17 And he is before all things, and in him all things hold together. 18 And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent. 19 For in him all the fullness of God was pleased to dwell, 20 and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross. (ESV emphasis added)

And of course the verse quoted in the body of the question - Hebrews 1:1-2 - which is merely the preface to a whole epistle dedicated to expounding the pre-eminence of Christ's revelation above all other revelators and His service to the Father above that of all other servants of God.

Protestants would affirm that God still speaks through the proclamation of the Gospel and the ministry of the word (preaching, teaching the 'whole counsel of God' etc.), but that these ministries don't add anything 'new' to the revelation of God in Christ - if they do, then they exceed 'sola scriptura' and can (and should!) be judged as error if not heresy.

There is some controversy within Protestantism between Cessationist and Continuationist camps on the additional nuance of this issue regarding ongoing 'special revelation', with (some) Cessationists asserting that anything of that nature (e.g. prophecies, dreams, visions, hearing the audible voice of God etc.) being not only un-necessary given the completed canon of scripture, but anti-thetical to 'sola scriptura' even if (as the near unanimity of Continuationists assert should be the case) such revelations are subject to testing according to the revelation of Christ drawn from scripture. And by the by, (most) Continuationists think Cessationists are actually willfully ignoring what the scriptures actually say about these issues and are hypocritically basing their doctrine in this area on extra-biblical arguments!

The greatest divergence from the Catholic perspective would be that what constitutes a complete understanding of God's revelation through Christ should not be mediated through the lens of the tradition and an 'infallible' teaching magesterium, but only through using the bible itself - Interpreting Scripture by Scripture.

So you see, even though most orthodox Christians would agree that Jesus is the 'final word' of God's revelation, there is plenty of scope for disagreement as to what this actually means, though it would be safe to say that most parties, though manifesting their polity in a variety of ecclestiastical forms, strive to accord with the pattern of Ephesians 4:4-16 -

4 There is one body and one Spirit—just as you were called to the one hope that belongs to your call— 5 one Lord, one faith, one baptism, 6 one God and Father of all, who is over all and through all and in all. 7 But grace was given to each one of us according to the measure of Christ's gift. 8 Therefore it says,

“When he ascended on high he led a host of captives, and he gave gifts to men.”

9 (In saying, “He ascended,” what does it mean but that he had also descended into the lower regions, the earth? 10 He who descended is the one who also ascended far above all the heavens, that he might fill all things.) 11 And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, 12 to equip the saints for the work of ministry, for building up the body of Christ, 13 until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, 14 so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. 15 Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ, 16 from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love. (ESV)

The final word refers to the final revelation, in practical terms Matthew to Revelation. There is no Scripture after this or above it. In the past God 'gradually' over a long period revealed his word, bit-by-bit, shadow-by-shadow but when Christ came he brought the final conclusion in the gospel of grace and truth.

For the law was given through Moses; grace and truth came through Jesus Christ. (NIV, Jn 1:17 )

And

In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe. (NIV, Heb 1:1-2)

In the same short period he also established all remaining doctrines necessary for the church through the Apostles and forever closed up into the canon of scripture.

This final revelation was initiated at the time of Christ ending very shortly after. Its glory and superiority is based on the directness of its communication in that God himself through the incarnation spoke it to men as a man. We are not to expect any new revelation and supplemental books such as the Koran, or other books various Christian sects have claimed to have been given as a supplement in the last few hundred years.