Speaker grills on or off?

It depends on the speaker. I leave them on if there's too much high frequency energy. My keepers all are used with grills off.

I actually think mine sound better with them off. There's more energy and clarity on the high end. Not a huge amount, but noticable. But, that said, it's very rare that I actually bother to remove them. They're magnetized, and very literally could not be easier to remove and/or replace. Let this be a testament to my lazyness.

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When listening, grilles off. When not playing, grilles on.

I don't know if there really is a difference, but just in case the grilles arent absolutely acoustically transparent, I keep them off when listening seriously.

On. No huge difference either way.

Agree that it depends on the speaker. My bookshelves sound better off, more dynamic. In main system, floorstanders are noticeably more cohesive with grills on.