Any "screen/no-screen" difference measurements?
"Acoustically transparent" is a claim that should be tested.
Using the trusty old Radio Shack SPL meter and an audio test disk revealed that there wasn’t more than about 3db of change to any frequency I tested for the speakers alone, with the screen, or even with the screen, secondary black backing layer AND masking panels.* Of course, the accuracy of my meter has an error of margin and is not as accurate as I wanted, but my ears heard no objections during music listening and movie watching as noted below.
I don't think the phrase "acoustically transparent" is meant to be taken literally. It just indicates that loudspeakers can be be placed behind the screen with little appreciable impact on the loudspeakers' FR. I doubt that there is any fabric on this planet that can make that claim in the literal sense - for screens or speaker grills.
I bought the Seymour XD fabric and Fidelio black velvet for a 104" screen and built my own frame. Even with shipping to Canada, the whole thing cost me $500. Granted, I bought rough wood, planed, routed and cut it to size myself. But, if you have the equipment and some basic skills, it's a great way to save money. When I was planning my setup, I asked my A/V dealer about AT screens and he said that to order a ready-made screen from him would cost about $2500!
I am very pleased with my screen and wouldn't hesitate to recommend Seymour.