The day I watched in horror one of my meticulously restored Quad ESLs arc. Through the grill, I could see a little spark walk across the treble panel, like a burning fuse on a stick of dynamite.
To my surprise, this didn't affect the sound all that much. I learned from the factory that small arcs, in aggregate, have a cumulative effect on the sound, but a single arc won't necessarily be disastrous. At worst, it may make the panel more vulnerable to future arc events.
Good thing I had a supply of NOS panels in the closet...