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

The meters:  Instantaneous is far superior to averaging.  A common misconception is the color of the backlite illumination matters.  This is a myth.

One might also include OTL. 

Looks….If the amp looks ugly it will play as such. LOL

My experience is that the color of the housing is the most influential determinant. A shiney black case produces less harmonic distortion. And I agree that a good on/off switch can make a critical difference. 

In SET amplifier:

1. Output tube.

2. Driver tube.

3. Coupling between stages (transformers and capacitors).

4. Cathodes (cathode or fixed bias, cathode bypass capacitors size and quality, resistors).

5. Signal transformers quality. Output, interstage, input.

6. Power supply. Fist capacitor quality. B+ capacitors quality and size. Voltage stabilization. Power transformers and chokes.