At any point did the speaker wires (the wires themselves, the terminals or whatever aligator clips, etc. you are using) make physical contact with one another while the pin's power was on? A photo of the connection to the cabinet speaker wires and another of the connection to the sub would provide a lot of information. I would not swap (i.e. risk) another sound board until we get a good look at your setup. thx

You say you ran the cabinet wires to the low-level inputs - is there a speaker level input rather than a low-level input? If you are running from the cabinet speaker wires you would most likely want something along the lines of a speaker level or hi-level input to the sub. Your sub may or may not have both hi and low options. Perhaps you can post a photo of the wire connection to the subwoofer - that would go a long way. thx