I have a 4 channel JBL amplifier, and I tried bridging the negative front left and the positive rear left to the crossover using the LP Input. The other input is HP Input. The components are a 3-way set, and I was not getting any bass out of the 6.5" woofer. I then tried bi-amping the crossover running the front left to the HP Input and the rear left to the LP Input, and everything worked fine. What am I doing wrong when I try to bridge it?

Thanks.