The center channel shake was at very high volume. It was caused by too much bass going into it and causing the speaker to vibrate.
You can still use the 6th channel in direct input mode.
i sometimes enjoy the 6th channel but I find that it takes too much away from the l/r surrounds. Even if I raise the level of the l/r surrounds, they still do not pack the punch that they do when in 5 channel mode.