Speaker grills

Off for listening, makes a difference in the upper end on my Proacs
and because they look much much better (I like to see the speakers, am I the only one?).

On at other times, just for dust protection.

I leave mine on my Verity Fidelio Encores. They take just an ever-so-slight hard edge off the treble.

I think keeping them on render a more neutral sound, sometimes taming a overall to bright amp
using proac studio 140