Speaker grills

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

Grills are constantly on. One pair of speakers cannot be removed, one pair was designed with grilles in place, and the other pair I prefer the look and sound of having the grilles on.
+1 to Mapman’s statement.