Another insight is to match your 12types during purchase. Whether you select Mullards or Telefunkens just make sure the vendor tests all the tubes as matched pairs. Same goes for KT88 you want a matched Quad of KT88/6550 regardless if it's NOS Genalex or GE, JJ/Telsa Blue Glass, Electro Harmonix, EAT, or SED.
FYI. Last year I knew nothing about tube rolling. Nothing!! But I was forced in to action because I started to hear popping sounds from the stock tubes and then my fuse blew days later.
Several phone calls later and hours reading on the web (GON and tube sites) I had answers to my questions.
What is your preamp and loudspeakers?
