There is a huge unknown here.
Are you using a preamplifier to receive the signals from the source devices and send them to the amplifiers?
Are you using a passive subwoofer? (If you are using an active subwoofer it already has an amplifier built in).
Does your subwoofer have a crossover switch or knob to adjust the output frequency? (commonly the dial will range from 20Hz to 100Hz).
Do your amplifiers have a loop/line level out?
Do your amplifiers have any crossover adjustments?
These all make a difference in the set up. If you can elaborate on the equipment we can provide a solution.
markc at orangeproav dot com