Ok, there's definitely something severely wrong with the amp.
I've just listened to "Sand" by Nancy Sinatra and Lee Hazlewood and this strange thing happened:
I can barely hear
his voice, while
her voice is normally audible. So I switched to the internal speakers of the iMac and there, both voices are audible just fine. I already mentioned that I'm not quite satisfied with the sound, but I didn't hear such an extreme effect until now.
Now I'm totally unsure if the amp always sounded like that or if it got worse over time.
But I guess it means that I'm switching to the internal speakers now and don't use the S.E.X. until it's fixed. I hope I didn't cause any permanent damage by listening to it that often.
Besides the measurements that you already suggested, where should I start?
My voltages were all a bit high, but still in the tolerance.