I finally got around to measuring Moleskin and Seymour center stage XD material.
the measured spectrum was 2kH to 20kHz, basically covering only my compression driver. I think anything below should not be significantly affected by either material and it is much easier to gate the measurements if low frequencies are out of the equation.
I have to note that my CD rolls of heavily after 12kHz so it is difficult to identify any effects at higher frequencies.
I could not use the same makeshift frame for both fabrics because it was not strong enough to hold the Moleskin stretched. I'll try to do more controlled measurements some time this week.
My first results look very promising for Moleskin.
The acoustical effect of the two fabrics is different - Center Stage causes comb filtering and Moleskin attenuates the sound with minimal or no comb filtering, probably because it has more uniform texture. AFAIK
simple signal attenuation is a better problem to have, it can be easily fixed by EQ. CB on the other hand is not curable (though the CB I observed with Seymour is likely inaudible any way).
the level of attenuation by Moleskin is depended on how stretched it is. When it's almost lose it was approx -3db across the measured spectrum. When I made it tighter it became almost fully transparent. I fact the results looked better than with the grille cover!
Since I could not use the frame for Moleskin I simply draped the whole speaker, tightly wrapping the waveguide with it.
When used on the screen there will be a t least few inches distance between the speaker and the fabric. This may change acoustic properties, making it more reflective, especially off-axis. That's why I want to repeat the measurements using a frame.
But I have to say I am pleasantly surprised by what I am seeing so far.