Why do amps sound different?

Hi folks, can anyone tell me why amps sound different? I know this is a very trivial question, but it isn't so trivial as I previously thought. For example: an amp can sound "warm", while the other can sound "lean" and a bit "cooler". These amps measure the same on the test bench, but why do they sound different? What causes the "warm" characteristic if the amp has pretty good measurements and frequency characteristics? It is certainly not measurable high frequency roll off, otherwise the amp sucks. Maybe one of the experts among us can elucidate this issue a bit. Thank you.

Chris