All you need is a bi-polar capacitor in series with each of your speakers. Assuming roughly a 4 ohm impedance (although at 80 to 100 hz. you are not too far away from speaker resonance), your value for the capacitor would be approx. 450 microfarad. Like I mentioned though, being this close to speaker resonance, I would personally cut that value in half. Buy two 225 microfarad caps or three 150 microfarad caps and put them in parallel to achieve 450 microfarad. That way you can peel some back if you wish to lower your value (raise the crossover point) to protect the speaker. Keep in mind this is a simple 1st order (6 db per octave) filter and therefore has limited protection capacity due to its gentle rolloff slope. Use your ears. They are some of your best tools when choosing a ballance between sound pressure and sound quality.