Hello.
The Z906 uses bare wires for the center, right? Does the rear of your Z906 look like this? :
If yes, you can basically use ANY 4 to 8ohm speaker as your center speaker. It can be a speaker from a vehicle, or a home speaker. As long as it has between 4-8 ohms, and your ideal size.
Simply connect the positive terminal of a new speaker to the red side of the rear panel of the Z906, where it is written "center", and the negative terminal of the new speaker to the black side,