Have a look at the Projects section on the ESP Audio Pages ( http://www.sound.au.com ). There are several active filter designs here, along with a programme for calculating component values for different crossover frequencies.
if you go to the Microchip website (www.microchip.com), you can download a free program called FilterLab that will design the filter for you... all you need to do is drop in your desired cutoff frequency, filter order and type. You can also fiddle with component values to get what you want. I would personally go with something like a 2nd order Butterworth, maybe 3rd if you're worried about passing higher frequencies through your sub.