I *think* I know what you mean. But just to be sure I have to ask: if the power supply ground is connected to the amplifier ground, and the amplifier ground is connected to the next device's ground, then no voltage difference can appear between them, right?
Are you saying that there is a common high point in this circuit? Or is this something about floating grounds which I haven't learned yet?
Sorry if these seem like basic questions (they are, I know) but I'm still working on gathering my theory (and experience).
Torben