nbdisperfectcorrection: use the following cable adaptor combination:
you plug the rca connectors to your sub, the other end to the adaptor below, then connect the wires jbl provides you to the lsr305 monitors and the other 3.5 mm end to that adaptor that you plug in your computer. The bottom line is the sub needs to share the inputs with the monitors due to the way the sub is designed. Those monitors are built to use their LSR310S sub which segregates the sub and satellites (LSR305) separately. That would be the best connection possible.