I can't comment on the caps going out of spec, but I believe that it was a notch filter, not a brick wall filter, in the SL600s, as the heavy, copper dome had its primary, oil can, resonance at the top of the audio range. The filter was deleted in the later, lighter, aluminum domes of the SL6S and SL700 that pushed the primary resonance further from the accepted audio range. The woofers were made of a proprietary plastic dubbed Cobex. The speakers were anything but bright, so if they are now, something has gone awry. Though the listening axis is higher than the other Celestions, an 18" stand being recommended, as opposed to the 24" stands of the 700 and SL6. It may be that the drivers do not sum correctly without ear height being above the tweeter. So this may be something to try.
IMHO although Richard Heyser's laser interferometry is often referred to as a "breakthrough" it certainly drew on prior art. In the '40s a flashlight and fan were used to study cone breakup modes in a very simplified way that clearly is conceptually related, yet infinitely more crude.
IMHO although Richard Heyser's laser interferometry is often referred to as a "breakthrough" it certainly drew on prior art. In the '40s a flashlight and fan were used to study cone breakup modes in a very simplified way that clearly is conceptually related, yet infinitely more crude.