Sorry for the late reply. But it addresses Logan's later post too.
Since the content of the "hymn" is not identified, we can only presume they sang a psalm, not prove it was a psalm. Was it likely they sang a Psalm? Sure. But even if it were true, there's still no command to sing Psalms exclusively.
Again, you are assuming the point that needs to proven. While it might be unthinkable to someone who already holds to EP that Christ would sing anything else, the text itself doesn't say that. In the scenes of heaven in Isaiah and Revelation, they are not singing Psalms. And why wouldn't Christ sing the Doxology? Or the Gloria Patri? Aren't those praises just as biblically true as a Psalm? If they accurately teach and admonish the inspired Word, what possible objection would Jesus have to singing them? In Reformed theology, we understand the preaching of the Word to be “the Word” when preached accurately. Why would the same principle not be true in singing an exposition of the Word? What evidence is there that Paul meant something different when referring to “teaching and admonition” through preaching vs. singing?
The headings in the LXX still don't prove EP. For the sake of argument, let's grant that Paul is saying to teach and admonish through the instrument of songs. What other vocabulary words would he use to describe these other songs other the words "psalms", "hymns." or "spiritual songs"? The EP argument is again assuming what needs to be proven. Paul himself did not restrict these terms to the Book of Psalms. Again, Paul knew how to say "Book of Psalms" or "The Psalms". That vocabulary was standard usage in the apostle's day as evidenced throughout the NT. Paul quoted from "the Psalms" often when quoting Scripture. But he didn't use it in the specific context of instructing Gentiles how or what to sing, Gentiles who came from a background of pagan hymnody. That consideration needs to be given due weight.
This is a key misinterpretation used in the EP argument. "Spiritual" does not mean "inspired". "Inspired" is a different Greek word. In context, in both Colossians and Ephesians, Paul is contrasting life in Christ vs. the earthly or fleshly life outside of Christ. These songs that expound "the word of Christ" are characteristic of our new spiritual life in Christ, and constantly drill that "Word of Christ" deeper into our hearts. Songs that praise Christ are "spiritual" because they teach and admonish the Word to God's people, and don't teach something else. They reinforce our life in the Spirit. There is no command anywhere to sing only "inspired songs". That is a category mistake, (just like the earlier misuse in this thread of the doctrine of "sufficiency"), applying the doctrine of inspiration in a way the Bible itself does not do.
Again, there is nothing in the context to indicate that Paul is commanding only the Book of Psalms to be used. The grammar of the passage indicates he is commanding the use of songs as a method of "teaching and admonishing" the word of Christ with "wisdom", the exact same vocabulary used to describe the act of preaching Christ earlier. Will the Psalms do that? Sure. But Paul did not limit teaching possibilities to the Book of Psalms only, nor did anyone else in the NT. Further, when Paul uses that term "the word" in both Colossians and Ephesians, he used it in reference to the fuller revelation of Christ made in the NT (Col. 1;5, 1:25, 4:3, Eph 1;13, 5:26). He had a bigger source of revelation in mind than just the book of Psalms or even the OT. There's no indication in the text anywhere that he has somehow narrowed down his concept of "the word" to "the Book of Psalms" when it comes to singing. Obviously they did sing Psalms a lot because it was already available, but there is no restriction given on what it means to teach with songs in the NT other than that they wisely teach the Word.
I love singing the Psalms, and lead my own congregation to do so every Sunday. But we can't make the Bible say what it doesn't say. It doesn't say "Book of Psalms only", but "teach and admonish" with songs, songs that teach the fuller revelation of "the word" to our people. To limit that singing only to the book of Psalms unnecessarily limits your ability to teach the Word because it restricts from our praise the more explicit vocabulary and concepts revealed later in the NT about our God and our salvation. Paul did not make such limitations in preaching, and there's no indication he did so with singing either.
And just to clarify for the rest of those on this thread, denying EP does not mean you are advocating for the use normative principle of worship. We are arguing that the regulative principle requires singing songs that teach the Word and help it dwell in us richly, not singing the Book of Psalms only. The difference is not over the regulative principle of worship, but about what it actually commands. There is no explicit command for exclusive psalmody, and I do not see how we can arrive at it by good and necessary consequence in the text of the NT. That's why I gave up the EP position years ago, after vigorously arguing for it (as my early posts on the Board will show). Once I better understood both Greek and the regulative principle of worship, I couldn't in good conscience bind my congregation to exclusive psalmody. There's just no command for it and God's people lose the use of beautiful songs that rightly expound the word of truth if we follow EP.
Another two cents...
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