Micro cracks in the glass. And I thought I'd heard everything.
The hotter a piece gets the more likely you will destroy your bank account by leaving it on. Amplifiers should always be turned off. I have a friend who always left his system on. His house got hit by lightening, blew his amp and destroyed his Wilsons. Worse, he did not have replacement value insurance so they prorated his stuff. He has been downsized to Vandersteens. If the amp had been off the Wilson's would have been just fine.