Hebrews 1:5 is a reference to Jesus and is quoting 2 Samuel 7:14. The speaker at 2 Samuel 7:14 is necessary the Father since Jesus is the son of God the Father who says, "I will be a father to him and he will be a son to me." David responds, and makes it clear he is responding to THIS same speaker's promise by saying, "there is none like you... there is no God besides you." v. 7:22. See also 1 Chronicles 17:14ff.