My Mesa Buster combo has blown a fuse twice in the last eight days. Both times I've replaced it immediately, and it's fine the rest of the night. Neither time was at high volume. I recently purchased it used, and I have no reason to believe that it has ever been abused. It still has the original power tubes (the cheap Chinese ones), and I've heard that the vibrations can shorten the life of the tubes. I was considering replacing the tubes with Svetlana's in the next few months. Should I go ahead and replace the tubes now?