@noromance Peter Walker said in an HiFi News interview that the felt on the back of the tweeter panel was to dampen a resonance problem around 2khz. And the burlap reduced back wave interference with the front radiation. HiFi News did a test with the 57's placed on 24 inch stands, with burlap removed. Sound quality was noted as more "open", particularly in bass/midrange. So I tried the same on my first pair, including removing the felt. Sounded quite good!