Power tubes glowing?

I've been listening to some stuff at pretty high volumes lately- enough to make your ears ring- and the last couple of nights, the ceramic insulators on my 845B tubes actually glow orange. You can't see it when it's daylight, or when the lights are on, but when it's dark, and I power everything down, the ceramic insulation is glowing orange hot. I know these tubes can run up to 200 degrees F, but should the insulation glow? Doesn't sound like it's clipping or overdriving, and I biased the tubes about a month ago. Something to worry about? Will it shorten tube life? Or perhaps I should re-bias the power tubes? Any ideas? Or am I fretting about nothing here?