Hi Shadyj, thanks for the reply. It offered me a new insight, and I looked at the back of my computer. The motherboard connectors look like this:
(I have the non-audio ports grayed out on purpose in this picture)
Currently, I have one 3.5mm jack going from the hole labeled "Front" to the old amplifier, which in turn is connected to four small speakers and a small subwoofer using RCA cables. The old amplifier does not have any more holes in it for me to add any more speakers to it.
Do you think I should just connect the new subwoofer to the hole labeled "Ctr Bass"? To do that I'd need the two items I described in the first post, of course.
Or it makes more sense to connect the new subwoofer to the hole labeled "Coaxial SPDIF"? In that case I only need to use the male to male RCA mono cable.