try reversing the wires on one side of the box, Hook positive to positive and negative to negative on one side of the box, and positive to negative and negative to positive on the other side of the box. many times with a bandpass enclosure of this sort it will increase output by putting the woofers in a push-pull configuration.
also be aware that the amplifier you are using is a home theater amplifier and is sending a full range signal to your woofer, not a properly crossed over bass only signal. for best results try to find a subwoofer amplifier (search for a plate style home theater subwoofer amplifier) to get the best out of your subs. also also, this enclosure is not going to be the best choice for a home theater setup, but if it makes you happy, have fun with it