Bthmed

Thats the female plug, you'll need a male also for the cable. You can also just jumper a piece of wire across to test and see if you like the second speaker before you start drilling holes in your speaker.

8 ohm speakers are very common and come in all sorts of shapes and sizes. Usually ohms is posted somewhere or in the owners manual that comes with it.

How big you get sort of depends on what your after. I just added a second one scavaged from another logitech. Radio shack has some small (about 3") 8 ohms speakers (home theater type) and I think you can get them with the housing also.

If you just want a bit more mid range/volume and better monitoring for yourself the above works fine. Gary's not here to advise, but I sort of got the Idea he was going to do away with the logitech center speaker and just go to a larger (5" to maybe 8") single center speaker which will probably be great.
I would rather have one on each side of me for better hearing for me. You can even put a large 15" combo club speaker 8 ohm in its place and blow yourself out of the room.

Hope this helps you some..........Pose