crossover question for subwoof

I have a set of "passive" subwoofs that are internally wired for a 12db rolloff at 38 cycles. When I hook them up to my main system (which is a two-way playing full-range), their is too much bass overlap from 38hz to about 200hz. I do not want to rolloff the main speakers, but i would like to rolloff the bass info from the subwoof in the 40hz to 200hz range. Basically just get the subwoof to play from 20hz to 40hz Anyone know if this is possible?