Reddit reviews Parts Express 100 Hz Low Pass 4 Ohm Crossover

Yes, you will need a passive crossover to run a sub and those speakers from a 2 channel amp. Is the sub a single voice coil? To put it another way, does it have a single pair of speaker wire connectors? If so, then your options are somewhat limited, BUT you could run the 6.5" speakers full range, then put a low-pass crossover on the sub, like this: http://www.amazon.com/Parts-Express-100-Pass-Crossover/dp/B004GE68N4/ will pass the low frequencies < 100 Hz to your sub. I'm not saying to buy that one, you will need to to buy that matches your sub's impedance (8,4,2 ohms)

not sure how much it will help, but here goes. the amp will have 3 cords coming out: left, right, ground. left goes to left speaker, right goes to right, and ground goes to both. who goes to the sub?

you would need a low pass filter. im not sure if you'd combine the left and right wires, then go into the filter (someone else is going to have to chime in). after that, you'll need a low pass filter, so you are only telling the woofer to play low notes. you'd also have ground go to the woofer.

this MIGHT be what you're looking for:
http://www.amazon.com/100-Low-Pass-Ohm-Crossover/dp/B004GE68N4/ref=sr_1_1?ie=UTF8&qid=1334952064&sr=8-1

edit: or this http://www.amazon.com/Pyramid-CR66-Electronic-Pass-Crossover/dp/B000A8B1TG/ref=sr_1_2?ie=UTF8&qid=1334952064&sr=8-2