I have had my beloved Invader now for almost three years and have decided to treat it to a new set of tubes. I love how the amp sounds so I have no hesitation replacing the tubes like for like with another set of Engl's own tubes.. My question being.. The EL34 powertubes in my Engl already have a label that states that they are "Grade 29." The shop that I am looking to buy the replacements from offer a "Grade 17" or a "Grade 32." What is the "grade" of an Engl tube? Does it matter which I buy? They are both the same price so it doesn't seem to be a "quality rating" so to speak... .. Thanks in advance for any help!