Okay, with so many opinions it seems no one here actually knows the answer to the question. Uh boy!
What contributes most to a change in how an amplifier sounds?
Amplifiers include tubes (if not solid state), big transformers, lots of internal wiring, Power supply, cabinet, gain controls if you're lucky, connections for incoming and outgoing cables, Computer chips, Control panels, semiconductor boards, design choices, age, etc.
Of all this stuff, what contributes the most to a change in how an amplifier sounds?