Thanks for the close-up photos of the resonator, @sorrodje! My current mod that @purrin measured a while ago employs some cloth around the back side of the driver, but not directly on top of the circle piece, as I felt that that added too much treble. But maybe the cloth I'm using is just about the same as what Senn is using, and maybe... just maybe... it's actually that that's dropping 6KHz, instead of just the foam thing? Just thinking out loud. Will see if I can replicate the resonator's effect without having to do anything destructive to the mesh/grill in front of the driver.