You mentioned a Pioneer SX2 controller, but without knowing the make and model of the existing mixer and speakers, I can't give any specific hookup recommendations. If you can come back to tell us what is in place, then we can give exact instructions.
There's a great page with lots of information about the Sub1/Sub2, and links to the owner's manual.
Let me know what's in place, I'll be happy to help!
The mixer they have is a old generic 8 channel mixer, same as the speakers; generic no name 15' PA's. It has 2 TS/TRS inputs for the PAs, that is it. I normally plug my 2 xlr's to any of the 8 channels and add a small bass via the RCA master output from the controller. That bass is too small hence my near purchase of the SUB1.
Thanks for getting back with some info on the existing system, but I still have very important questions, whether the speakers are "powered" or "passive", and whether the mixer is powered or not? (Pictures of the speakers and mixer, both front and back, would help greatly.)
If the speakers are powered, they will have two cables running to them, one for the signal from the mixer and the other providing electric power. They each have their own internal amplifier, and will usually have an on/off switch and an indicator light to let you know they are turned on. Powered speakers are used with an unpowered mixer, which is usually relatively thin and light, often with the knobs and/or sliders on top.
If the speakers are passive, they will only have one heavier speaker cable (generally a heavier gauge than a mic cable) that carries an amplified signal from either a separate amplifier, or a mixer with internal amplifier (i.e., a powered mixer.) The speakers are usually lighter, since they don't have an internal amplifier, and won't have an on/off switch or indicator light. Powered mixers are usually larger/thicker and heavier (sometimes shaped like a box) to accommodate the internal amplifier.
If you are using powered speakers, the Sub1/Sub2 connection is fairly simple. Run two cables from the mixer's main outs to the two "LINE IN" jacks on the Sub, then run two XLR-TRS cables from the Sub's "LINE OUT" jacks to the speakers. On the rear of the Sub, press the "LINE IN EQ" button so the "LPF" light is lit. For the "LINE OUT EQ", there are two settings: One is for "HPF" (high-pass filter), which will send only the higher frequencies to the 15" speakers; the other is "FULL", which will send the entire frequency range. Depending on the design and performance of your existing speakers, you'll need to try both settings to see which sounds better.
If you are using passive speakers, you cannot connect the Sub directly to them; doing so would risk damage to both systems. You would need to verify whether there are one or more separate "line out" (signal only, unpowered) jacks on the mixer that you can send to the Sub. This is where pictures of the mixer would be most helpful.