i can see, that it could be read as though the subject that 'testified' is what the Spirit in the prophets 'signified' -- so that 'it' would fit.
i could also see, that it could be read as though the subject testifying is the Spirit that was also signifying -- so 'He' would fit.
so overall my comment on that subject is just that i think it's not a big doctrinal difference; it's equivalent in both KJV and NASB. i tried to say that earlier but maybe i could phrase it better now: it's like this, if i quote some passage of scripture i could say,
the Bible says " . . . . "
or i could say
God says " . . . . "
and there is no material difference between those two ways of saying it. the Bible would take the English pronoun '
it' and God in English would take the pronoun '
He' -- i could say it says; i could say He says. whichever way doesn't change the meaning of what i do by quoting the Bible.
overall, my view of the broad subject, is that language is a medium for communicating ideas & concepts.
those concepts/ideas are the thing that is written or spoken; that is what God has preserved on earth. when a person reads or hears the Bible, no matter who they are and what language they speak and what time period they are living in, concepts & ideas are being communicated to them by the Spirit who breathed them. someone in China isn't hearing the same 'words' made up of sounds or reading the same 'words' signified by marks on a paper when they read John 1, but they are reading the same
thing being communicated to them by the same
Spirit. so 'the Word' of God, the scripture, isn't 'text' at all -- it's more than that. it's something transcendent, that we as humans use oral & written language to describe; language in and of itself is only a tangible representation of a metaphysical thing that is communicated.
example: blue.
note that the text is colored black, but the idea isn't