VAC has autobias, the IQ system, in their newer models and protective warning leds. If the led turns green that tube is getting weak, if it turns red, the tube is drawing excessive current and the IQ system shuts down the main power supply in a fraction of a second (from the manual) so that you can replace the bad tube before it does any damage.
I don’t know which autobiasing and tube protection system works better, nor do I know which brand sounds better.