Sorry, I should've explained it better.
Best example I can give is the stereo in my truck. I have a deck, or receiver, tuner...It has 4 speaker outputs at like 25 watts per channel. It also has 5 RCA outputs. The 4 speaker outputs, I don't even use. My deck essentially generates no power or wattage. Just sends low volt signals to the amplifiers. I have a 4 channel amp that runs the door speakers. That's where four of the RCA's go. The amp is filtered for mids and highs.. The 5th is for a sub woofer amp which drives two 10"s under the backseat. Filtered for lows.
In the P5, I have the overhead system that has two powered 6" built in. Has RCA outputs for a sub. That's what you basically have too. I put in a tube sub which has a built in amplifier. Look up Powered Bazooka Tube.
You just need to pick what you want on the other end. You can run a separate amp and woofer but it takes up more room. What roc has is a powered shallow mount woofer. Made to go under a car seat. Which wouldn't be a bad place if you could find one to fit. It's not gonna hit very hard but a little low end is better than nothing.