First, is it possible to send a signal from my XLS1000 to a powered subwoofer? If yes, how would I do that? I see that a loop through signal can be sent out, but I don't understand how that works. Thanks.
You need a 'Y' cable. Find two balanced wires. Strip the two wire 1" at each end. Strip individual wires for an appropriate length. Connect (solder) the two wires at one end.
You now have two parallel wires joined at one end; The tied side goes to the signal source; each loose end then goes to the inputs of the amps. Put appropriate mating connectors on each end.