There are four instances of the aorist participle that appear in Phil. 2:7-8, namely, λαβών, γενόμενος, εὑρεθεὶς, and a second occurrence of γενόμενος. Donald Mastronarde points out that the aorist participle "conveys the aspect of the aorist stem, that is, simple occurrence or completion of an action" and "most often refers to an action antecedent to that of the main verb of the sentence and is usually translated in English by a past participle." So in the case of the four aorist participles presently under discussion, they would literally be rendered in English: 'having taken,' 'having become,' having been found,' and 'having become.' λαβών would thus possibly refer to an action antecedent to the main verb ἐκένωσεν--hence, it would not describe the instrumentality of the "emptying" delineated in Phil. 2:7.
Moises Silva also makes the same observation in his commentary on Philippians, when he writes: "If we were to press this rule [for aorist participles], we would need to translate each clause [in Phil. 2:7-8] somewhat woodenly: for example, 'having taken (or, after he had taken) the form of a servant' " (Silva 120). But Silva then goes to say that this rule has many exceptions and he is right (see Mastronarde 199ff). But I think that he is possibly wrong when he goes on to insist that the best way to handle the aorist participles λαβών and γενόμενος in Phil. 2:7 is to treat them as participles of means. It seems that both Silva and Daniel B. Wallace primarily favor the participle of means in Phil. 2:7 (outside of theological considerations) because of the supposed "poetic quality" of Philippians. Yet this "quality" is questionable since this verse might not be derived from an older hymn as commonly supposed.
Furthermore, construing λαβών as a participle of means does not do justice to ἐκένωσεν, which normally denotes an act of negation (subtraction), not the act of addition (as even Wallace admits). So the NWT rendering seems to make the most sense out of the context and it is well within the parameters of Greek grammar when it translates Phil. 2:7: "No, but he emptied himself and took a slave's form and came to be in the likeness of men."
"No, but he emptied himself and took a slave's form and became human" (2013 Revision).
"But himself emptied, taking a servant's form" (Rotherham).
At one time, I favored the antecedent view for the participle λαβών in Phil 2:7. After talking with a friend and studying Stanley Porter's Idioms, however, I must concede that λαβών may be referring to consequent action or, more likely, subsequent action. At any rate, it is by no means certain that the aorist verbal adjective here is a participle of means. While categorizing Phil 2:7 as an instance of the instrumental participle (i.e., participle of means), Brooks and Winbery admit that λαβών and γενόμενος "may indicate manner rather than means" (Syntax of NT Greek, 150). To be fair, they also say that there's little difference between a participle of manner and one of means.
It also just dawned on me that NWT renders 2:7 polysyndetically, but Rotherham and some others, render the verse asyndetically.