My Focal speakers have this black grill that I can put on my speakers. My question is do you find much of a difference with the grills being on or off speakers? To me it seems the high end is a little diminished with them on. Granted every company’s covers are a little different, but it seems it may be best to play music without them. Do you agree, or do you feel the protection against dust, etc makes it worth leaving them on.
I have had speakers where the Mfg. Recommended them on as they were tested that way. My current mfg. Recommends them off.
I would think that if they are so inhibitive that you can hear a difference, then i would be tempted to leave them on for dust protection etc but remove them for a “session”.
I just leave mine on. Speakers look better, a bit of protection and i really cant tell a difference
My current FR-30 and previous Silverline M.C. speakers, the SQ is much better with grills removed.
On every speaker I’ve owned the difference between grills on and off has been clearly audible to me. As with just about everything in audio, the correct action is to listen to the one you like the best.
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I have Focals too, they make a very big difference to my ears.
I have Focals, and after comparing the sound quality between grills on and off, once I took the grills off, they never went back on.
Harbeth say that their grilles reduce treble by about 1dB, but their speakers are voiced with grilles in place, so for the most accurate reproduction they should not be removed.
Obviously, it is a matter of taste. If you want it a little brighter, cats scratching your drivers, and grandchildren pushing pencils and Lego bricks into the ports, then by all means listen to the bare speaker 
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My old Richard Allens had foam “couching” under the grill cloth on the baffle board.
It was without doubt the most attractive-to-cats object I ever owned - slowly and steadily destroyed over several years.
Must have been very satisfying for them.
Current pair of cats (acquired in January) have shown no interest in any of my speakers, nor indeed any of the music I play 
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