Welcome to the forums and hopefully, your interest in sound reinforcement will keep you here for awhile....there's plenty of material to soak up.....
is currently configured in monitor/main mode, the mains are 2 custom built 3-way speakers and there are 2 monitors also. The system has a great sound and has performed well for us.
Our question: Is it possible to add a subwoofer to this setup, and if so, what would be the proper proceedure. (Rewiring the speakers, crossovers, series/parallel wiring?)
I am not sure about the 684 model, but I would think that you have pretty much maxed out your system. You might be able to add a powered sub through an out of some sort. I do not know if the "outs" on the front ALSO disable the link to the internal power amps. If they do not, then you could hook up a powered sub with a low pass filter (like a PV-118D) to the Left or Right out.