Carver’s challenge that he could make a solid state amp sound like any other amp, or like a tube amp? I imagine that it would not be very hard to take the electrical characteristics of one amp and engineer another amp to measure similarly in the areas that would affect sound quality.
It was to make any amp sound like any amp, though he did so by taking measurements and engineering a circuit with various knobs and switches that allowed him to adjust the distortion and frequency response in various ways.
He didn't say all amps sound the same, only that you can make a cheap amp sound like an expensive one.