This thread might be old,
But it sure did help accomplish what I was looking for, the suggestions above using the 2.0 or using a 2.1 set to fill in the missing center works great!
Here are some detailed info:
In your sound card, if you are using the Logitech Z560 default setup
you'll have the green (front left + right speakers) connected to the "Green" jack in your sound card,
and then have black (rear left + right speakers) connected to the "Black" jack in your sound card,
so to fill in the missing center speaker, you hook up your set of 2.0 or 2.1 speakers into the "Orange" hole which drives the center speaker & sub-woofer.
Now you'll probably think wouldn't it better to have the Logitech Z560 in the "Orange" to make better use of the 500w sub, well I tried that and it didn't give the result you'll expect.
Apparently the Logitech Z560 wasn't designed for the Sub to be running independently on its own channel, so when you connect either the Green plug, or Black plug into the Orange jack, all it does is convert the Left / Right into a Center + Sub, which will sound rather horrible as a Sub replacement.
So it's best to keep the Logitech Z560 THX 4.1 as is, as the 500w Sub still works even after you plug in a 2.1 into the Orange jack.
Pros
- separated volume for the center channel
- no electrical hacking required to your equipment.
Cons
- 2 volume knobs to control
- the 2.1 / 2.0 speaker set needs to be powered,
I'll suggest to use good quality speakers that produces mid-range or full-range sound to match the sound quality from the Logitech satellite speakers
When connecting the 2.0 or 2.1 set into the Orange Jack, you don't really need both Left + Right speakers
If your speaker set has detachable wires, then you only need to use the one that has the Power / Volume control, so technically you are only doing 1.0 or 1.1 to fill in the center speaker.
If you plug in both Left + Right speakers, all it does is play the base / Sub's sound through the extra speaker, and if the speaker is not of good quality the vibration makes it sounds horrible, reason why I suggested to use good quality speakers.
Alright, that's all,
Have fun and enjoy your games/movies @ 5.1
AG