Hi Mike
Well,the way I understand it works is that the signal from the power amp goes to your main speakers.The left channel signal goes to the left speaker with the positive and the negative wires and gets split up at the internal crossover of the speaker.
The low ,mid and high frequencies are routed to the corresponding drivers.Same with the right channel.Now to add a subwoofer .The subs usually come with their own exclusive power amp and crossover.Basically their power amp is a stripped down power amp ,because it is not required to reproduce full range frequencies but only the lows.Say below 120Hz or something.My sub is of that design and has it's own amp so it gets connected to the line out(tape loop) of my pre-amp to get signal feed.Now there are subs with different connection but I think you need an out board crossover or something.Also there are complete three piece systems but the crossover is housed somewhere to do the signal split.
I am not an expert here,though.If the sub you mentioned comes with a build in crossover then you probably can feed it a signal from your power amp.It all depends on the design I guess.I hope this helps .Most likely you were aware of it anyway.
Later
George