What contributes most to a change in how an amplifier sounds?

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.

A good design  helps a lot. 

Output impedience.

If you follow the graphics in Stereophile, you see that the higher the output impedience, the greater the variation in frequency response based on a real life speaker load that is not a simple resistor.

The loudness button.

The transformer is the beginning to great sound.  It has to handle the requirements of a recording.  If you think about it, why do different amplification from the same manufacturer sound different or handle music differently?  Quick Silver just stopped making one of their amplifiers because they could not source the transformers any longer.  We used to use Plitron transformers but they now require a minimum order of 100 pieces.  Designs are not new so depending on what you are looking for, a design is important to the sound and cost.  Power supply is the key to great sound.  That needs to filter AC noise, handle dynamic swings, etc.  Then capacitors and resistors have an impact.  Tubes vs. SS is personal preference.

Happy Listening.