My guess is that it's the low order harmonics of (most) tube amps (rather than any roll-off per se) that give them that golden, velvety, sepia-tinted glow.
(Most) solid state amps exhibit the opposite, nasty grating sound of high order harmonics, odd-order harmonics and crossover distortion.
(Most) solid state amps exhibit the opposite, nasty grating sound of high order harmonics, odd-order harmonics and crossover distortion.