The way I consider the "Secret Chiefs" at present is that they (and other astral masters) are personifications of the Current of the group in question (in this case, the Golden Dawn). There is a definite "sentience" to a magical Current, which means that there is definitive Briatic consciousness, for want of a better term. However, we would project our own Yetziratic personae onto that Briatic consciousness, and the Current would actively facilitate this. Thus, the heart of the message is transmitted, but the vessel of it, the form it takes (including language choice and grammar) is dependent entirely on the receiver. This explains why grammatical errors of old English and the like show up in channelled works.
So, the Secret Chiefs are, in a sense, metaphorical. They are a way of describing what is a somewhat abstract form of contact with the Current, a kind of magical "collective consciousness", to borrow Jung's terminology. Indeed, I believe the older Rosicrucian groups originated the term "Secret Chiefs" (roughly in the 1700s, from the German unbekannte Oberen), and, like most elements of Rosicrucian teaching, it is meant to be allegorical, not literal. However, there is a reality behind the allegory - except it is much less fanciful, and anyone with a true connection and a receptive vessel can establish this contact with the Current - otherwise known in personified form as the "Secret Chiefs".