The spot in front of the rectifiers is where I apply DC because mine came with a red and black wire harness there and the other was yellow and black. Mine doesn't increase power when you increase the voltage there, however. It will decrease the power if you go below 10 volts, though. I don't have a transformer to apply the correct AC voltage to it, so I haven't tried that. I did measure the diode ratio between the two plugs and you can measure the diodes between two of the pins I am using and one of the other pins that I have come to assume is the DC input.
But, I have not pried off all the glue to gain access to the back of the board, so I can't say for certain that it is the DC input. It was also because someone else was using that as the DC input, IIRC.