There are a lot of mobile electronic crossovers available with prices that are all over the map. If you're on a budget, or just want to try one to see if it's really what you need, check out the 3-way crossover at Radio Shack, P/N 12-2011. It's on sale right now for $40 (normally $50). It provides outputs for low, mid and high frequencies. I believe the crossover points are adjustable. I've never tried one myself, so I have no practical experience with it - looks like a nice unit though.
At $40, it's a real bargain because if you were to use high quality capacitors to filter out the lows, the cost would almost equal the Rat Shack unit.
I'm surprised your Zapco doesn't already have an electronic filter in it to pass only frequencies below 120Hz or so.
-Wes
