You probably have an issue with only certain frequencies, sounds like higher voice frequencies, resonating in your room somehow. The parametric equalization detected that and minimized those frequencies. Such problems can have a very narrow frequency range, which is why you don't notice it on all voices, or even the same voice all the time.
Clearly Alan would be better to answer this, but my understanding is that you have two options; placement or equalization. (there's the third option of acoustic treatment for the room, but for most of us that isn't an option.) Have you tried under the TV, angling up?
Bruce
