Coen ,

As far as I'm concerned you've nailed it ( a bad batch of diaphragms , ( metal related ) ; too thin, wrong Alu mix, wrong metal prep, etc. etc. ) .

Distortion in that elevated area is particularly noticeable to the human ear ( & I really have no idea what internet based cool-aid Vuki is drinking when denying the audiblilty of distortion / I have my suspicions that it starts and ends with the initials E.G. ) .

I'd love to see some sweeps of your 802s ( Fr & Dist ) on the same 511Es ( that would be a learning experience for all of us ) . When you have time .

Thanks for the educational thread ( now you know & now we know ) ><