Grills ON or Grills OFF?

My current speakers are designed w/o grills. Prior to that my Dynaudios and Vienna Acoustics sounded best with the grills 'OFF'. Grills should only be used for shipping to protect the drivers from packing mishaps or young children.

I can honestly say that I don't notice any difference in SQ between grills on or grills off. But, having said that, I prefer grills off. However, I notice that i'm indifferent when using my system for movies. However, for whatever reason, when listening to music I much prefer the grills off. Just seem to enjoy the listening session more when I'm looking at the actual speakers.

What's a grill?

You know, one of those things you cook on at tailgate parties.

It depends if the grill was an after thought by the designer or if like on Vandersteens it was part of the design. It is not the shear Fabric we hear but the diffraction caused by most grill frames. Vandersteen speaakers use the grill and frames to smooth the transistion from the driver edges and the enclosure.
Without them large peaks and dips + - 6 DB can be heard and measured. If in dought call the manufacture and ask if the speakers were designed with the grills on or off. Look at the pictures of show reports and see if the manufactures showed with them or without.