To me, the Psalms declare the salvation of all in a hidden manner, when one looks at their beginning and end.
Psalm 1 begins with a man, who does not walk in the counsel of the wicked, or stand in the way of sinners, but delights in the law of the Lord. This man is Jesus Christ, likened to that Tree of Life, planted by the waters and yielding it's fruit in season. Even as salvation begins with One man, the Christ, so do the the Psalms.
Psalm 150 is the final Psalm, and ends with a chorus of universal blessedness. "Let everything that has breath, praise the Lord!" This shows the 1 Cor 15 finality of the salvation process, when all things are subject to Christ, and Christ is subject to God, that God may be All in All.