The more I dig in, the more I think the EP position is probably right.
EP'ers, thank you for your patience. I think you won a "convert", if such language can be employed on this issue.
The rest of you - I am still waiting for the command to sing non-psalms. I find the three suggestions brought up here so far unconvincing.
I'm one of those who have already responded to this subject many times and don't wish to get wrapped up in another thread, but i will give you my reasons.
The simple reason to oppose EP is that there is no command to sing only the Psalter. This is what caused me to leave behind the EP position. The RPW requires an explicit command to make something an element of worship. We have a clear command to sing. We do not have a clear command to sing "the Book of Psalms".
Paul knew how to say "the Book of Psalms" (Lk 20:42, Acts 1:20) and chose not to use it. It was in the conventional usage of his day. And whatever little was written about NT singing, it never commands the "Book of Psalms". Instead, we are commanded to "teach and admonish with wisdom" the Word of Christ with songs, the same words used for preaching (Col 1:28), and that if anyone has a hymn he should use it for edification (1 Cor 14:26). In short, we are commanded to use songs to teach the Word and edify. That is the command to write new songs.
The EP argument that God required "inspired songs" is based upon a misunderstanding of "spiritual songs". Spiritual does not mean inspired. Spiritual is used many times to refer to "spiritual matters" or to our spiritual life in Christ compared to the earthly or worldly existence of those outside of Christ, a theme Paul dealt with both in Col 3 and Eph 5 prior to the commands about songs. We are singing about spiritual truths in contrast to earthly things.
Furthermore, using the categories of "man-made songs" vs. "inspired songs" is not a category used within the Bible itself. Those are man-made categories being imposed on the text. "Inspiration" refers to process in which Scripture was written down by the prophets. It's not used in discussing what songs to sing.
Why command to teach with songs? Because with the coming of Christ, greater revelation of the mysteries of the gospel were revealed and needed to be taught to the Church, through preaching and song. The book of Psalms as part of the Old Covenant scriptures were sufficient for worship under the old covenant, but did not adequately reveal or explain those later mysteries which are now crucial for us to know and believe, which is why we have a New Testament along with pastors/teachers to further explain them.
Further, if EP were commanded, then why doesn't Paul make such an important element of worship clear when writing to Gentile audiences, when all it took was using the standard title "Book of Psalms" instead of the more cryptic "psalms, hymns, and spiritual songs"? He (or any other apostle) only had to use it one time in reference to singing, and it would have been clear. When viewed within the broader context of Paul's emphasis on teaching the Word in Colossians, Ephesians, and even 1 Corinthians, we can see that songs are commanded as a means of teaching the fuller revelation of Christ to the Church.
The early church obviously sang Psalms, and I believe we should sing them too. But we are not commanded to sing them exclusively. We are commanded to teach the Word with more songs. Just as preaching and teaching requires exposition of the the Word, so does the use of songs.
There are many more complex arguments dealing with the composition of the book of Psalms and what place they held in OT worship, which Ian Duguid has answered before in other threads. So I won't rehash them here.
But there's my two cents.