John, I don't know if it makes any sense other than 0 being an easy to remember reference point(for example, on the receiver that I mentioned, as best I can tell 60 is the "unity gain" point, but I'm not really sure). But as I understand it, if for example the input is 0.1V and the amplifier section gain is the typical 29dB, then the overall gain at 0 is 29dB and as a result the 0.1V is amplified to about 2.83V,[20log(2.83/0.1)] which is of course the standard voltage for the speaker sensitivity 1 meter spec.
