Why don't you just use a multimeter to see which of the two leads is connected to the tip of the jack? That would be your positive lead and the sleeve would of course be ground. Unplug the jack from the amp and test it. I know it's sort of a simple answer but it's the right one and I should have thought perhaps the obvious one...
:BEER
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Because using a multimeter won't tell you which lead is positive on the speaker itself, only on the jack, which isn't the problem.
Either way, there is enough info here to solve it.
Either way, there is enough info here to solve it.