Got a 1967 NOS GE smoked glass 6V6 tube in the mail today. Swapped it for the Chinese tube in my Champion 600. Definitely an improvement over stock! Seems louder and to have more clarity. Have a Mullard 12AX7 coming for the preamp. It's been mentioned here that replacing the preamp tube will make an even more noticeable difference than the 6V6 swap. Can't wait to try it and see what comes. Should be here sometime this week hopefully.