But as bigsby said there be voltages running through the head even when not powered up as it be stored. But generally you would change all the tubes so they all have the same life (yet some may blow before others even when brand new) you don't have to have the same brand but the same type of tube yes, unless it is set to be able to change tube types. My head currently has a set up where it has different brands best for doing certain jobs. You will need to bias it to get the best out of the tubes, if running too hot then it can work the tubes to hard, but if set too low then not getting the optimum performance out of the tubes. Get a matched set, at least you know the tubes be set the same (again depends on quality control) But if you don't know what your doing then don't throw your hands in there. I've always done my tubes and biasing but always used electronics and feel comfortable doing it.