While ἐκ + the genitive might be employed by a writer to "more sharply define" the partitive nuance of the head noun, this construction is by no means required for a particular construction to be partitive (or "wholative"). Daniel B. Wallace in fact shows that the partitive genitive "is a phenomenological use of the genitive that requires the head noun to have a lexical nuance indicating portion" (Greek Grammar Beyond the Basics, page 84).
καὶ τὸ δέκατον τῆς πόλεως ἔπεσεν (Revelation 11:13).
There are a number of other factors involved when one is trying to identify which constructions are instances of the partitive genitive. See Wallace, pp. 84-86.
We also find an excellent discussion of the partitive genitive in David Aune's commentary on Revelation (pp. Volume 1: clxxi-clxxiii).
He demonstrates that John uses the partitive genitive as object of the verb in three distinct ways:
(1) With the simple genitive.
(2) With the preposition ἐκ + the genitive.
(3) Coupled with the preposition ἀπό + the genitive.
I also encourage you to consult C. F. D. Moule's discussion concerning the partitive genitive in An Idiom of New Testament Greek (Cambridge: Cambridge University Press, 1990). Vide pp. 42-43.
One example Moule provides in his short treatment of the partitive genitive is Romans 15:26: κοινωνίαν τινὰ ποιήσασθαι εἰς τοὺς πτωχοὺς τῶν ἁγίων τῶν ἐν Ἱερουσαλήμ.
My contention is that there's no doubt πρωτότοκος πάσης κτίσεως could be an instance of the partitive genitive. Whether one chooses to understand Colossians 1:15 as such is another matter, but the use or non-use of ἐκ + the genitive does not wholly determine whether the construction is partitive or not. The context as well as lexical semantics must decide the question.
ὅς ἐστιν εἰκὼν τοῦ θεοῦ τοῦ ἀοράτου, πρωτότοκος πάσης κτίσεως (Colossians 1:15 W-H)
καὶ αὐτός ἐστιν ἡ κεφαλὴ τοῦ σώματος, τῆς ἐκκλησίας· ὅς ἐστιν ἡ ἀρχή, πρωτότοκος ἐκ τῶν νεκρῶν, ἵνα γένηται ἐν πᾶσιν αὐτὸς πρωτεύων (Colossians 1:18 W-H)
Some want to make much of the fact that the Greek preposition ἐκ appears in 1:18, but not in 1:15.
ἐκ in 1:18 could be used to emphasize Jesus' resurrection from the dead, as one friend of mine has suggested; but there is another explanation that may also account for ἐκ without resorting to a Trinitarian alternative.
Petr Pokorny (Colossians. Peabody, MA: Hendrickson, 1991, page 84) writes in ftn. 153 concerning 1:18:
"MSS P46, Aleph (first hand), and others omit ἐκ = from. The sentence reads the same way in Rev 1:5. The meaning is not altered thereby."
Revelation 1:5 has ὁ πρωτότοκος τῶν νεκρῶν. It evidently means the same thing that Colossians' πρωτότοκος ἐκ τῶν νεκρῶν does, as the MSS evidence indicates. It just seems that ἐκ is normally used when the resurrection of Jesus Christ is under consideration. See John 21:14; Rom 4:24; 6:4; 10:7; Col 2:12; Gal 1:1; 1 Pet 1:3, 21.
Meyer's NT Commentary on Colossians 1:18: "comp. Revelation 1:5, where the partitive genitive τῶν νεκρ. (not ἐκ. τ. ν.) yields a form of conceiving the matter not materially different."