to with the speaker grille on. Some other manufacturers recommend listening with them off. If they come off easily so as not to damage them I don't see why one shouldn't try them off. If it sounds better, why not?

yeah, i like to listen to them off. they sound more open and the treble seems better somehow. with them on they sound different . with them on i think the treble mid and bass might be more cohesive.  it seems from a lay persons point of view that you would want them off because it seems like the sound waves would hit the fabric somehow and diffract differenly then they would if the grill were off and the sound waves were alowed to travel "directly to you". or do sound waves go straight through the fabric?

I haven't seen any test data backing those up. I do know that measurements revealed a difference in the Large Advent speakers, but those speakers are 40 years old.

There is no general answer.  Some are designed to be used seriously with the grilles off and some with the grilles on.  Moreover, some people prefer to listen oppositely.  

For an example of speaker measurements which show that the grille should be left on, see:  http://www.stereophile.com/content/paradigm-reference-studio60-v3-loudspeaker-measurements

Kal

There does seem to be a bit more openness with the grills off, plus I think the speakers look better with them off.  However, I have four cats so when I am not around I put the grills back on, and lean a 42" x 10" piece of plastic (garage door window) against the grill.

Try both ways.  I have one set of speakers where they sound better off and the other set sounds better on.

I think the protection is more important than any effect on sound.