I don’t give quite as much weight to what the Reformers said as Scott Clark, nor do I think his conclusions mean we must sing only psalms, but his explanation of “psalms, hymns, and spiritual songs” is exactly what I teach my students. Here’s a snippet:
In short, when the Reformed thought about “psalms, hymns, and spiritual songs” in Colossians and Ephesians, given their background in the Latin Bible, they would not have interpreted that phrase as “the canonical psalms, a type of non-canonical song, and another type of non-canonical song.” The evidence from the Latin Bible simply will not allow such an interpretation. When they read those nouns, they understood them against the background of their use in the Latin Bible. When they read “psalms, hymns, and spiritual songs” in Colossians and Ephesians they read them as types of Psalms or as synonyms for Psalms or other typological forms of praise.