When amp tubes need to be replaced 6lf6's ect. ect.. Do thay need to be of the same brand? Can thay be mixed and matched? Assuming thay all test 80-100% does it matter which company made them or if there used or nos? I'm lucky that my 300a's output is at the rated power. Output is as it stated in the manual. Im assuming this means the tube are of a good to high %age. Curiositys got me.