OMG, just removed my speaker grills and the sound just opened up to a new level!

Vandersteen are designed to be played grills on 

removed every grill on every speaker I’ve owned since 1987, and they all sounded the same on or off.

   Cool story.

Vandersteen are designed to be played grills on 

Yep, had to remove the ‘socks’ of my 2CE Sigs while getting one of the acoustic couplers repaired. I know what they looked like without, but just plain ugly naked when seeing them for myself. Of course, some find them ugly anyway ;-)

But yes, Richard designed/voiced them with the socks on, as that is how they are inherently designed. I’ve grown to actually like them, regardless.

Funny, I’ve always left my speaker grills on, mostly out of fear of damage, and also dust, as I’ve always lived in older homes. 

Were the grills made of wool or placebium? 

Grills tend to affect the treble the most and works as an attenuation. If a speaker was intentionally voiced with the grills on then it will probably sound "better" with the grill on. Basically they probably add 1db or so to the treble so to counteract the attenuation caused by the grills.

The higher order affect of the grill is a little bit less apparent. A grill works like a filter, albeit a mechanical one, therefore it will filter out more and more toward the higher frequencies. That is the grill will attenuate the high frequencies but NOT by a fixed factor, but gradually toward the high frequencies. For example, at 10KHz, the attenuation is 1db, whereas at 15KHz it would be 1.5db, and at 20KHz by 2db.  And like any filter, the grill will add phase shift to the higher frequencies and phase shift generally is not a good thing.  A fixed 1db added on the tweeter probably will not undo the damage caused by the grill. It may help a little bit but won’t fix everything.

Regardless of the treble energy, removing the grill will most likely add some openess to the sound. By the way, "more trebles" does not mean "more openess".