This question has been asked in every audiophile forum and it’s interesting to see that so far more people than I usually see are preferring grills on.
My preference is: Grills on.
With caveats.
The reason I generally prefer grills on is that I’m generally not a big fan of seeing all the technology producing the sound - to me very often, speaker drivers are like exposed to plumbing. And when I see the speaker drivers my mind helplessly maps the sound I’m hearing onto those drivers. “ those drum symbols are coming from that tweeter right there.”
Once grills have been placed covering the drivers, for me that seems to sort of relax my brain and then speaker seems more like a piece of furniture around which the music is just occurring.
Another thing is, I tend to find more speakers actually sound a bit more coherent with the grills on.
On the other hand: so many audiophiles listen without the speaker grill as the default that many if not most speaker manufacturers seem to design for grills off listening (if they even supply a grill). So that often means the speakers sound best without grills.
But what it also means aesthetically is that when speaker manufacturers supply grills, they look more like an afterthought “ hear these things just in case you want to use them.” And they tend to stick awkwardly on the speaker looking more like a Band-Aid placed over the driver’s rather than integrating the look of the grills.
So I don’t like ugly grills either.
Fortunately one of my favourite loudspeakers that I own are the Thiel 2.7s, which were designed to sound correct with the grills on, and we’re also designed aesthetically with the grills in mind, so that the inset grills fit flush with the front baffle, making for a very deliberate cohesive look IMO:
My Joseph audio speakers have grills that I keep on when I’m not listening. They aren’t as coherent looking as the Thiel grills, but for “ stick them on her off” type grills they aren’t too bad:
I like the sound with the grills on, but they definitely need the grills off to get that final bit of airy magic and timbral beauty.
Fortunately, I find the SEAS graphene drivers (and the tweeter with the copper ring) or actually some of the finer looking drivers, they have a very high-quality materials finish that I can appreciate:
But I often listen with lights down on the speakers, so I end up not seeing the drivers while music is playing, while getting the best sound from the speakers: