at least there was now continuity between two of its pins with no plug inserted, unlike before

Are you sure that you haven't connected the wrong two pins together and shorted the power supply, or that it wasn't the power supply that was faulty in the first place?
Might be worth putting the inside of your Empeg back the way it was, borrowing a 12v power supply and seeing if it works then (Make sure that you get the polarity right).
....other than which, I step aside for the FAQ master!