σὸν … δὴ is more poetical and more impressive than σοὶ … δεῖ: cp. 197,
: Ph. 15 “ἀλλ᾽ ἔργον ἤδη σὸν τὰ λοίφ᾽ ὑπηρετεῖν”: Aesch. Theb. 232 “σὸν δ᾽ αὖ τὸ σιγᾶν”. But “σοὶ...δεῖ”, though a rare, is an admissible construction; besides Eur. Hipp. 940 (quoted on 570) cp. Xen. Anab. 3.4.35 “δεῖ ἐπισάξαι τὸν ἵππον Πέρσῃ ἀνδρὶ καὶ χαλινῶσαι δεῖ”: Mem. 3. 3. 10 “εἴ σοι δέοι διδάσκειν”: Oecon. 7. 20 “δεῖ μέντοι τοῖς μέλλουσιν ἀνθρώποις ἕξειν ὅ τι εἰσφέρωσιν”: ib. 8. 9 “εἰ...διαλέγειν δέοι αὐτῷ”. We cannot read σοὶ … δὴ with L, and understand “πάρεστι”, as Campbell proposes. φαίνειν τὰ λαμπρὰ ἔπη = φαίνειν τὰς ἀρετὰς δι᾽ ἃς ἐπαινεῖσθε, to illustrate the praises by deeds: cp. Od. 8.237 “ἀλλ᾽ ἐθέλεις ἀρετὴν σὴν φαινέμεν ἥ τοι ὀπηδεῖ”. φαίνειν ἔπη could not mean strictly “βεβαιοῦν ἔπη”, to "make" the words "good."