Hello bocapimp,
A few possible thoughts.
1. Check all of your speaker wire connections. It is possible a channel (such as you center channel speaker wire) has come loose. You'll want to check this at both the speaker and the amplifier.
2. You may also want to perform a system reset, though keep in mind that this function will erase all of your stored settings, so yuo may wish to write them down before hand so that your Model 990 may be reconfigured quickly.
Scott