This kind of sounds like the reverse of the roll-off in THX systems where the highs in sounds in the movie theater are adapted for home use since the same highs are boosted because the speakers are behind a screen in the theater. I would think a grill cloth over the speakers would certainly have a noticeable effect in somewhat deadening the sound coming out of the tweeter especially if it is producing a flat response curve.
With a speaker the size of the VP180, dialog intelligibility should NOT be an issue. Obviously, something else is going on here.
