Securing your precious drivers from kids' busy fingers and cats' sharp claws can be trouble! SBAcoustics has taken the trouble serious and done something about it and made magnetic grills that stick to your mounting screws. A nice wooden frame with small neo magnets and nice grill cloth.
But how do they work and what impact will it have on frequency response - in particular in the treble range where diffraction is an issue.
Here some results of testing the grills on a tweeter, a 4½ inch midrange and 6" midrange.

My testing panel (80 x 120 cm) and the grills.

The MR13 midrange w/wo grill.

Tweeter TW29BN-B-4:

Above TW29BN-B-4 with and without grill. I don't think I have to state which is which.
Due to some serious diffraction we have huge dip around 7 kHz.
I have tried the tweeter grills on some other speaker and this is clearly audible.

TW29RN-B-4 w/wo grill, horizontal dispersion.
Red: No grill, on-axis.
Green: With grill, on-axis.
Yellow: With grill, 10 deg. off-axis.
Orange: With grill, 20 deg. off-axis.
Purple: with grill, 30 deg. off-axis.
As can be seen, on-axis makes the worst case, but not so much as we move off-axis.

Midrange, MR13P-8

The midrange MR13P-8 (edge-coated) as-is (red) and with grill on-axis (red).
With a point of crossover below 3 kHz, we may not experience much difference.
I don't have any speaker with said mid-driver, so I can't evaluate sonically.


MR13P-8 (edge-coated) horizontal dispersion with no grill.
0-10-20-30-40 deg. off-axis (red, green, etc.)

Same as above with front grill.

Midrange MR16P-8 (edge-coated)

MR16P-8 (edge-coated) w/wo grill, red and green.
Unlikely to cause much impact with a point of crossover below 3 kHz. 

Same as above with grill at 0, 10, 20, 30 deg. off-axis.