400W is only 1.5% louder than 380W - don't worry. I would recap this amp for better bass control. I changed power supply caps once and that is what I noticed. Bass became shorter more dynamic and cleaner.
Older caps have higher ESR (internal resistance) because they dry out. This internal resistance will keep increasing progressively because of increased self heating (but it will take many years). Eventually it will become so high that full power test, that you did, might trigger thermal runaway (temperature increases ESR that increases temperature by self heating even further and so on) and cap might blow up its mechanical fuse (rubber plug) or explode.
Electrolytic caps are the only components that age and are worth replacing if you like the amp. There is a good chance that you might be able to find today much better capacitors (lower ESR) than original.
Older caps have higher ESR (internal resistance) because they dry out. This internal resistance will keep increasing progressively because of increased self heating (but it will take many years). Eventually it will become so high that full power test, that you did, might trigger thermal runaway (temperature increases ESR that increases temperature by self heating even further and so on) and cap might blow up its mechanical fuse (rubber plug) or explode.
Electrolytic caps are the only components that age and are worth replacing if you like the amp. There is a good chance that you might be able to find today much better capacitors (lower ESR) than original.