[shifty]

Use an in-line capacitor, aka 'inductor', 'coil', 'PAC/low pass filter', any other number of terms are used to describe it. It discretely mounts inside your box and filters all highs and mids from your subwoofer (you willl likely damage your subwoofer if you keep cranking highs and mids through it).

Example: http://www.speakerboy.org/files/T3_Inductor.jpg

You can buy these at any respectable audio shop nearby for around $15 and it is an exceptional alternative to using a crossover (it is actually what they use in a crossover). Be sure to explain the imedance (ohms) of your subwoofer to the stereo guy.

While you're at it, I would recommend adding bass blockers to your small midrange speakers in the box. Here, these things, you need one per positive wire:

http://www.crutchfield.com/S-C2O9TUD...roup.asp?g=761