I have an odd situation with a 5 ch Power amp. Is the following abnormal? Currently I have the 3 front channels of this amp hooked up. It seems to play fine. However, when I power down the amp, source etc, I get a quiet but noticeable whine emanating from the center speaker. It seems to increase in pitch as it decreases in volume. It occurs maybe 10 sec after the amp is switched off. There are no sounds from the other 2 front channels, have not tried the surrounds. Is this a cause for concern?