How to hook up amplifier

I'm noticing that my Fisher speaks i have hooked into my smaller sub which is plugged into my Mac, are overpowering the sub. I have an old Onkyo amplifier lying around, and i wanted to try hooking my computer up to that, with the Fishers and sub. Now i know how to hook the Fishers and computer into the amplifier, but i don't know how to plug in the sub, because my past attempts haven't worked out yet. Does anyone have any input on this?