Just like an entire audio system, an amplifier is a chain of circuits and parts that’s only as good as it’s weakest link. At some level, everything matters, but many folks feel that it all starts with the power supply. In the case of tube amps, I can't help but think the quality of the output transformers is pretty important.
My amps started as Dynaco 70s, then got a boost in the power supply that helped. Later on they got modified with the VTA circuit boards that address the power supply and audio circuit even further, better caps, etc....a;; very audible improvement with the same output tubes and transformers. Later I changed the output and driver tubes, then upgraded the rectification to SS. All made audible changes that I felt were upgrades. When I finally bi-amped the system in a horizontal configuration, the Dyna/VTA amps went from seeing a 4 ohm load to an 8 ohm load where the tube amps only drive the midbass and tweeters, and that made another very audible improvement. All of these changes contribute to what I’m hearing now, but I can’t say that any one of them was more important than the others. They all add up, and they all matter.
FWIW, I’ve always preferred them in triode.
