Wolf, I would be glad to blame my power amps (Linn) for this, because I love the sound of my system (and my tube preamp as well (Audio Research)). I will send my Linns to assistence without specifying I use a tube preamp. Hope I will discover the cause of this damage.
About tubes I have to say I don't love too much tubes power amps, they lack the punch and the bass frequencies speed I love for my rock records and I had to change power tubes too often for my taste, (my integrated tube amp was a VTL, maybe it was a bad match with my speakers, who knows?) Anyway good tubes sound great if correctly matched. Tubes are like vinyl, they need some more care. I will inform you about the 'guilty'!