grilles on or off the speakers??

There are no grilles on my ESP speakers. The fabric covering is permament, so comparison is not possible. An obvious choice by the speaker designer to have you not listen without the fabric.
I prefer the appearance of my N802's with the grilles on and that's the way I listen. There is a difference (slight) between clothed and naked but I've become accustomed to the sound with the grilles on. Someone else may hear it the other way.
My Triad System 3 speakers are designed to be used with the grilles in place. The drivers are staggered in depth on the baffle and the grille provides the tweeter with a certain amount of controlled directivity. They don't sound right at all with the grilles removed.
So, my answer would be specially designed grilles are science and not snake oil. OTOH some grilles provide an enhanced appearance without undermining the performance of the speaker which I would also rate as good design.

I have Vienna Acoustic Bach Baby Grand speakers and after six months of listening without grilles, I put them back on. They stage a little more precisely, bass goes deeper and is more controlled, but mostly they seem more balanced top-to-bottom. And I am less nervous about my housekeeper dusting them every two weeks.

Ex_midwest
My brother in law has those same speakers and I will admit to me they sound better with the grills off (for most of what say with your grills on except for the bass). I guess it may also have to do with the total system synergy and the room they are in.

Interesting post I'll admit.

BTW my C1 grills are on when my grandsons come over (came way too close to loosing a tweeter because of a thrown wooden coaster).

the only good reason to leave grills off is the prevent the lousy veneer from fading everyw exhcept under the grills. i've seen speakers cabinets fade faster than you can imagine in rooms with average light.

The only reason to leave grills off is that almost every speaker sounds better without them. Try to imagine how obstructing the sound waves can improve the sound. My Mini Utopias did not come with any, have wooden shields that fit over them for protection when being moved.