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I'd try polystyrene glue on that diffusor plate. As pure Chemistry, we try to glue metal to plastic.
I tried it! Its' in place, but the grill looks bad with the irregular dry cloudy spot. I'm very fond of aesthetics, so I was thinking about removing the thing altogether. Would there be some predictable audible consequences?
I'm thinking about getting the suggested spare voice coil later on (that wouldn't require any adaptations to sound like the original, right?)