Whenever I try to increase the volume while watching DVD's, the voulme does not seem to increase above -15. Even if I crank it to -5, the volume level is pretty much the same at when it's set at -15. This only seem to happen when using DVD as a source. The volume seems to work well when I'm using CD as a source. I am using a separate NAD CD and DVD player. Is this a setting problem or what?