Yeah, we ought to have an unsolved mysteries category, it would be interested to see what the outcome turned out to be. I liked the one with the system hum that baffled the OP for quite a while and was finally traced/linked to the dc cabinet lights in the kitchen being on.
Ultrasonic Oscillation
Hi. I have had recent issues with my Snell Ds, tweeters not working. Anyway this led me to test of all of my equipment including my GMA Europas. Their tweets were dead also. I dropped an email to Roy at GMA and he suggested that I may consider the possibility of ultrasonic oscillation in either my amp or pre-amp. They are the Odyssey Candela pre amp, and Stratos amp. Could you please explain what it is, and more importantly what are the causes. Soon I'll get a diagnostic locally and sending the pieces to Klaus for repairs if needed. I look forward to your insights. Thanks much, Dave