I'm with Johnk on this one. The rational part of my brain thinks the wavelengths of a subwoofer's output would be much longer than the point at which diffraction effects would be audible. However, sub grills can darn sure rattle. I'd get a test tone cd like the ones Rives or Stereophile sells and check to make sure no grill buzzes need attention. I've had to use dabs of Blutack to damp the odd buzz in a sub grill.
