Hi Loose,
It depends on how well the grille has been designed. The Axiom grilles have negligible influence on frequency response. There is a tiny difference (a fraction of a dB) in the upper-octave response around 15 kHz with the grille on, but in double-blind tests, I've not been able to detect any audible difference using music as a test source.
With some brands, and a clumsy massive grille design, there can be significant audible difference with the grille on. Whatever speakers you own, it's easy to check. Take the grille off one speaker and leave it on the other; position the speakers fairly close to each other, perhaps a foot apart, and play some familiar music. Use the balance control on your receiver to go from one speaker back to the other. A crude test, but if there was a gross difference, you might hear it.
Regards,
