Personally, I can't hear much difference with my Athenas. However, I think the Audition series look really nice -- cleaner, and classier -- with the grilles on. I find when I take them off, the tweeters and woofers are very distracting visually. Just can't help but look at them every once in a while, even when playing a movie. For that reason, I've decided to leave 'em on. Just my two cents.
cheers,
supervij