Have you actually considered measuring their value ? I wouldn't mind betting
that the caps you're saying sound better have actually dried out and their value
is a fraction of the value printed on the can. That *would* affect the
amplifier's dynamics. You could mimic that effect by using new caps of lower
value than original.



The capacitance value usually stays pretty close, what happens is the
ESR increases, so a smaller value cap won't do it, but a resistor in
series will.