Subwoofers - Grille off or on?

Personally, my Dali Suite 1.2 subs sound best with the grills on. Go figure! My Spicas sound best with the grill on too!

I know speakers always sound best with the grilles off

The statement above is not accurate,sorry.

Does this apply for subs too, or does it not matter?

You have to experiment and decide for yourself.

I have never had a pair of full range speakers that sounded better with grills so I never use them. 3 of the 4 pairs of subs I have have no grill over the speaker, being either upward firing or downward firing. I do have grills on my Nelson Reed 1204s. Considering the frequencies involved I doubt that the grills would be audible; usually the problem with grills is treble attenuation, which of course does not apply here.

Wrong. It depends on the baffle design. Many speakers will definitely sound better with the grill on.

A grill on or off is irrelevant for a subwoofer and makes no difference.

I'm going out on a limb and siding with Shadorne on this one.

Marty