Hans, have you ruled out interconnect or speaker cables? You might try reversing everything going back to the source. If it is affecting both sides, you might try one speaker in another system.
Tweeter damage
Anyone heard damaged tweteers? Do they go lika a fuse, i.e. either they work or are totally corupt? Or can they just be slightly damaged? I send a Dolby Digital signal through analog pre-amp...A VERY unpleasent experience! Read that it could damage my ears as well as my speakers... Grateful for any info! Hans -Sweden (Hence my English)