Tube bear traps...

I've been doing a very small amount of tube rolling and for the first time I left the push down, spring loaded covers off because I was just getting annoyed taking 'em off over and over again. My impression was that they dampen the tube vibrations to keep feedback out. Why is it then that my amp sounds better without the covers on? I don't detect any feedback at all. It just sounds a bit more open and cleaner. Are the covers trapping too much heat and killing performance? Safe to leave them off?
Yes it's perfectly safe to leave them off. If your tubes start making noise at some point you can put them back on and see if they reduce the noise.