Words change, also meanings, don't they? We are talking about Hebrews 1:8, John 14:28, and the Trinity doctrine, aren't we? (The "throne.") Now I appreciate that you're interested in this, so let's try to stick to the subject.
At John 14:28, where Jesus said the Father is greater than I am but -- one would have to add interpretation in to that statement in order to insert it in the trinity doctrine. John 14:28 -- "the Father is GREATER than I am." There is more to say about Hebrews 1:8, and I see we're getting quite deep, at least I am, and for inquirers I think we should, but it can require a detailed discussion. And, since scholars themselves have disagreed, I guess each one will have to decide what he goes with or accepts.
One commentary (Meyer's New Testament Commentary) says this in part about the wording, or translation at Hebrews 1:8::
"it has been thought by Grimm (Theol. Literaturbl. to the Darmstadt Allg. Kirch.-Zeit. 1857, No. 29, p. 662) and Ewald (das Sendschr. and. Hebr. p. 55), ought not to be explained in the sense of a vocative. According to Grimm, the words are to be taken in the acceptation: “Thy throne, i.e. the foundation of Thy throne, is God;” according to Ewald, they say that “the throne of the Messiah for everlasting ages is God Himself, so that where He reigns, there God Himself is virtually ever present.” (That from Meyer's New Testament Commentary)