Speaker grills on or off?

i saw a report in the net years ago. it's an independent test using a microphone placed at a normal listening position, using scientific software and recording equipment to access the difference with speakers grills on and off. The results clearly reflects there are roll off in certain parts of the frequencies spectrum, not just in the treble or bass. 
So it does depends on the listener preference whether he like grills on or off. 
-phil

geoff, i could not remember that magazine that was put up in the net. it wasn't in english though. i used 3rd party app for translation, but the graphs are easy to understand.