Trying to Focus
First, let me compliment everyone on their contributions to an interesting audio thread, especially the OP for actually using test procedures instead of relying solely on subjective impressions. I also used to "do this stuff" pretty hardcore back in the '70s, studying with the techs at Seattle's Speakerlab and building some pretty high-end home systems, including custom speaker cabinets, custom cross-overs, Phase Linear amps, blah, blah.i went ahead and rebuilt iy but removed the grills, i might have to test them again and see if the sound changes any with the grills removed
So here's my small contribution, relating to the quote. Well-designed speaker grilles may improve dispersion of upper-midrange and treble frequencies. You should be able to hear a difference between the grill on and off. With a good grill, the higher frequency sound will be more consistent as you move your head around or change seating positions in the car or other listening environment. This helps in the localization or sound-staging of instruments.