As I understand it hitting the "green button" doesn't actually turn the unit on and off, it controls a servo which handles that job. This is a protection circuit used to either protect the unit or the speakers from recieving a sudden burst of energy. The pop is the mechanical sound of the servo in operation.
Many, if not all, recievers operate in this manner. The pop doesn't come through the speakers and may in fact be there to protect the speakers, though perception is such that you can fool yourself into thinking it does.

I am extreemly surprised that Gonk's unit does not pop when hooked up to the 750. This could mean that the servo is only there to "turn on" the speakers. Perhaps I will test that in a month or two.

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"Art is not a mirror, but a hammer."