An active crossover will allow you to adjust the sound level and frequency sent to the sub. It's been a long time since I've looked at crossovers, so I'm not sure which to recommend. Maybe someone else can chime in.
Edit: a quick search shows that even a small crossover is going to run about $50. You may be better off just getting a small 100-200 watt amp which would allow you to do the same with the added bonus of slightly more power. I've been out of the audiophile world for 15-20 years, so I'm probably not the best person to answer this.
