Speaker grills on or off?

Depends on the speakers. Some speakers such as the Thiel CS 2's were designed to be used with the grills on.

I agree that dome speakers were designed with them to stay on.

Here's a thought; why not one on, one off ;-)

I am surprised by the number of people who don't hear a difference. I am extremely skeptical of a lot of tweaks/ cable differences but I have always heard a difference when removing grills.

I have almost every model of speaker that John Dunlavy designed and they all sound better with the grills off, no exceptions.

No question. OFF