Speaker grills

Off when in use and on when not.

For Sennheiser HD600 headphones the answer is definitely without grills.

Off: When listening on the weekends! Or impressing fiends during the weak....

As a rule of thumb, most speakers sound better with the grills off. I have the optional grills for my Magico S5's & found this to be the case. However bare in mind some speakers have been designed such that the speaker grills are an integral part of the design. Two such examples would be the classic Infinity Renaissance series speakers & most of the current Avalon models. Ultimately you'd have to a-b test the speakers with and without the grills to know for sure.

Of course it should be mentioned that most grills are *extremely* transparent to the sound, so what doesn't make sense is that having no grills would subjectively make so much difference in the sound. But nobody ever experiments to see why a transparent grill is so bad for the sound. You could even make an experiment using a grill that has only say 1/5 of the number of strands of fibers so the grill is mostly open. Then it would be really, really difficult to explain, no? Same goes for headphone grills, they are bad news.