Poole on Revelation 5:9: In Praise of the Lamb!
Verse 9: And (Ps. 40:3; Rev. 14:3) they sung a new song, saying, (Rev. 4:11) Thou art worthy to take the book, and to open the seals thereof: (Rev. 5:6) for thou wast slain, and (Acts 20:28; Rom. 3:24; 1 Cor. 6:20; 7:23; Eph. 1:7; Col. 1:14; Heb. 9:12; 1 Pet. 1:18, 19; 2 Pet. 2:1; 1 John 1:7; Rev. 14:4) hast redeemed us to God by thy blood (Dan. 4:1; 6:25; Rev. 7:9; 11:9; 14:6) out of every kindred, and tongue, and people, and nation…
[And they sung a new song] That is, new to mortals, among whom such was never heard (Menochius, similarly Ribera); or, unfamiliar and wonderful (Menochius), not like the other songs that are in Scripture (Ribera, Menochius): or, new, on account of the freshness of the matter (Gomar), and fresh and pleasant matter for singing (Tirinus): or, excellent (Gomar, Piscator, Durham), which was able to be expressed by no old song (Gomar), and with singular joy (Piscator). New songs are more greatly esteemed (Grotius, similarly Forbes), says Pindar (Grotius), and Homer in The Odyssey 1 (Forbes). See Psalm 33:3; 40:3; 96:1; 149:1; Isaiah 42:10 (Grotius). New, that is, more open than under the Old Testament (Durham).
And they sung a new song: by a new song is either to be understood an excellent song, (for new songs are usually most valued,) or (which pleaseth me best) new as to the matter of it; for the servants of God under the Old Testament could not bless God for the actual redemption of man by the blood of Christ, but only rejoice in hope, embracing the promises seen afar off by the eye of faith.
[Thou art worthy, etc.] Worthy, who hast known and revealest the secrets of God; who hast suffered such great things for the honor of God, hast done such great things (Grotius). From here perhaps light is shed on that passage in Mark 13:32, No one knows, neither the Son, etc. Since the Apocalypse was not yet given to Christ by the Father, and the order of events to be conducted unto His coming was not yet revealed. I assert nothing rashly; let the reader consider it carefully (Mede’s Works 545).
Saying, Thou art worthy to take the book, and to open the seals thereof; they acknowledge Christ worthy to be intrusted with his church, and the revelations of the counsels of God, with relation to it, to open them.
[Because (which word denotes that Christ by the merit of His passion had received from God the power to reveal future events, etc. [Menochius]) thou wast slain] More is understood than is said. For, from this, that He is said to have been killed, it is to be understood that He offered Himself to death so that He might serve God and benefit men (Grotius). Therefore, exaltation is owed to Him (Ribera).
[And, etc., καὶ ἠγόρασας, etc.] And thou hast bought, or redeemed (that is, liberated from sin and the servitude of Satan [Gomar], and of the Law, and from the wrath of God [Durham]; and that at the redemption price [Gomar, thus Cluverus, Piscator], as it is evident from a comparison with 1 Corinthians 6:20; 7:23; 2 Peter 2:1; Revelation 14:3, 4 [Cluverus]) us (that is, Christians of all places and times, whom Christ liberated from sin, at great expense, that is, of His own life, Romans 3:24; 1 Corinthians 1:30; 1 Peter 1:18; Galatians 4:5; and elsewhere [Grotius]) to God (that is, so that we might be the possession of God [Menochius], εἰς λατρεύειν αὐτῷ, in order to serve God, Luke 1:74 [Grotius]) by thy blood (Piscator, etc.), since it was also the blood of God, Acts 20:28: blood is for life and breath, which Christ laid down for us (Cluverus). Hence it is evident that the living Creatures are not Angels, since they were not able to say, Thou hast redeemed us, etc. (certain interpreters in Gomar). Response: Understand us as a good part of our assembly, namely, blessed souls. There is a synecdoche of this sort in Matthew 27:44, compared with Luke 23:39 (Gomar).
[From every tribe, etc.] That is, the elect (Menochius). From all the tribes of Israel, and from men of so many tongues, whether they are the more civilized, who are wont to be called λαοὶ/peoples, like the Greeks and Italians; or Barbarians, to whom especially agrees the name ἐθνῶν/nations: it is λαοί, φυλαί, γλῶσσαι, peoples, tribes, tongues, in Daniel 3:4 (Grotius).
For thou wast slain, and hast redeemed us, etc.; because he had redeemed his church, scattered over all the world, from sin, death, and hell, unto God, to serve him, and to live for ever with him, and that with no less price than his own blood; Wherefore (as the apostle tells us, Philippians 2:9) God hath exalted him.
 Greek: καὶ ᾄδουσιν ᾠδὴν καινήν, λέγοντες, Ἄξιος εἶ λαβεῖν τὸ βιβλίον, καὶ ἀνοῖξαι τὰς σφραγῖδας αὐτοῦ· ὅτι ἐσφάγης, καὶ ἠγόρασας τῷ Θεῷ ἡμᾶς ἐν τῷ αἵματί σου ἐκ πάσης φυλῆς καὶ γλώσσης καὶ λαοῦ καὶ ἔθνους.  Pindar (522 BC-443 BC) was a lyric poet of Greece, esteemed by some to be the greatest.  Revelation 5:9b: “…and hast redeemed us to God by thy blood out of every kindred (φυλῆς/tribe), and tongue (γλώσσης), and people (λαοῦ), and nation (ἔθνους)…”  Daniel 3:4: “Then an herald cried aloud, To you it is commanded, O people, nations, and languages (עַֽמְמַיָּ֔א אֻמַּיָּ֖א וְלִשָּׁנַיָּֽא׃; λαοί φυλαί γλῶσσαι, in the Theodotion)…”