I don't follow your rationale - we are to "panta dokimazete" and keep the good, and not just the preaching and prayer.
God prescribes what is for His doxa, seeing as finite creatures cannot by searching find out God. God has prescribed the forms to be used in congregational singing, and has not given liberty for uninspired men to compose these forms and impose them on the congregation, for that would add to His word. He has given liberty to men to preach and to pray extempore, and for the congregation to judge whether or not said preaching and praying does in fact glorify God, according to the rule of the Word.