I am about to change out the tubes in my Tone King Comet 20 (ser #50). The tubes have never been changed over the 4 or so years I have owned the amp, and it has lately developed a loud random crackle which I am hoping will be cured by renewing the tubes (3 X 12ax7a, 2 X 6v6, 1 X 5ar4). I read somewhere that rectifier tubes rarely fail. Should I replace it anyway, or would that just be a waste of money? It's at least 4 years old. I use the amp twice a week for several hours at a time. Thanks.